24. REGIONAL GEOCHEMISTRY OF THE LAU …...more typical seafloor spreading fabric. The latter region is interpreted as having formed from the still active propagating spreading ridges

Hawkins, J., Parson, L., Allan, J., et al., 1994Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 135

24. REGIONAL GEOCHEMISTRY OF THE LAU-TONGA ARC AND BACKARC SYSTEMS1

A. Ewart,2 W.B. Bryan,3 B.W. Chappell,4 and R.L. Rudnick5

ABSTRACT

Detailed comparison of mineralogy, and major and trace geochemistry are presented for the modern Lau Basin spreadingcenters, the Sites 834-839 lavas, the modern Tonga-Kermadec arc volcanics, the northern Tongan boninites, and the Lau Ridgevolcanics. The data clearly confirm the variations from near normal mid-ocean-ridge basalt (N-MORB) chemistries (e.g., Site834, Central Lau Spreading Center) to strongly arc-like (e.g., Site 839, Valu Fa), the latter closely comparable to the modern arcvolcanoes. Sites 835 and 836 and the East Lau Spreading Center represent transitional chemistries. Bulk compositions range fromandesitic to basaltic, but lavas from Sites 834 and 836 and the Central Lau Spreading Center extend toward more silica-under-saturated compositions. The Valu Fa and modern Tonga-Kermadec arc lavas, in contrast, are dominated by basaltic andesites.The phenocryst and groundmass mineralogies show the strong arc-like affinities of the Site 839 lavas, which are also characterizedby the existence of very magnesian olivines (up to Fo90_92) and Cr-rich spinels in Units 3 and 6, and highly anorthitic plagioclasesin Units 2 and 9.

The regional patterns of mineralogical and geochemical variations are interpreted in terms of two competing processesaffecting the inferred magma sources: (1) mantle depletion processes, caused by previous melt extractions linked to backarcmagmatism, and (2) enrichment in large-ion-lithophüe elements, caused by a subduction contribution. Ageneral trend of increasingdepletion is inferred both eastward across the Lau Basin toward the modern arc, and northward along the Tongan (and Kermadec)Arc. Numerical modeling suggests that multistage magma extraction can explain the low abundances (relative to N-MORB) ofelements such as Nb, Ta, and Ti, known to be characteristic of island arc magmas. It is further suggested that a subduction jumpfollowing prolonged slab rollback could account for the initiation of the Lau Basin opening, plausibly allowing a later influx ofnew mantle, as required by the recognition of a two-stage opening of the Lau Basin.

INTRODUCTION

During the past decade, the Lau Basin and Tonga Ridge have beenexplored by a relatively large number of cruises, culminating in theOcean Drilling Project (ODP) Leg 135. Notable among these cruisesare those of the Thomas Washington (e.g., Volpe et al., 1988); S.P. Lee(Scholl et al., 1985; Vallier et al., 1991); Sonne in 1984/85 and againin 1987 (e.g., von Stackelberg and Shipboard Scientific Party, 1985,1988; Sunkel, 1990; Frenzel et al., 1990); Charles Darwin (e.g.,Parson et al., 1990; Collier and Sinha, 1990); Natsushima (Falloonand Crawford, 1991; Falloon et al., 1987); and Akademik MstislavKeldysh (Falloon et al., 1992). The results accumulated by thesecruises now allow the compilation of a good data base of petrological,geochemical, morphological, and geophysical data on which to de-velop models based on the recognition of patterns of geochemicalbehavior, interrelated temporally and geographically to the progres-sive opening of the Lau Basin. To these ends, the Leg 135 drill siteshave provided the invaluable additional dimension of timing.

Particular scientific interest in the active Lau Basin arises from therecognition of a range of compositions extending from near-normalmid-ocean-ridge basalt (N-MORB) in the central spreading ridges tobasalts enriched in large-ion-lithophile elements (LILEs) and light-rare-earth elements (LREEs), the so-called transitional or T-typebasalts (Hawkins and Melchior, 1985). More recent studies haveshown that the lavas within the southern Lau Basin, notably the ValuFa Ridge, have distinctly "arc-like" geochemical characteristics (e.g.,Sunkel, 1990; Boespflug et al, 1990; Vallier et al., 1991). Apart from

1 Hawkins, J., Parson, L., Allan, J., et al, 1994. Proc. ODP, Sci. Results, 135: CollegeStation, TX (Ocean Drilling Program).

2 Department of Earth Sciences, University of Queensland, St. Lucia, Queensland4072, Australia.

3 Woods Hole Oceanographic Institution, Department of Geology and Geophysics,Woods Hole, MA 02543, U.S.A.

4 Department of Geology, Australian National University.5 Research School of Earth Sciences, Australian National University, G.P.O. Box 4,

Canberra, ACT 2601, Australia.

considerations as to the origins of these variations, it is also necessaryto obtain a clearer geographic and temporal picture of these patternsof regional variations, and to compare them with the adjacent modernarc volcanoes of Tonga. Another relevant question is that of the status,if any, of a distinct backarc basin basalt type.

The purpose of this paper, therefore, is to bring together existinggeochemical data on the spreading centers of the Lau Basin, to comparethese data with the modern axial Tongan and Kermadec volcanoes (i.e.,the modern arc volcanic systems) and the older arc volcanics of theLau Ridge, and to relate these data further to the volcanic sequencesrecovered in Sites 834-839 of Leg 135. In addition, the boninites fromthe northern end of the Tonga Ridge are also included in these com-parisons, as these lavas clearly constitute an integral part of the arc andbackarc systems.

It is relevant to provide here a brief summary of the major volcanictypes encountered at Sites 834-839 (Parson, Hawkins, Allan, et al.,1992). Samples from Site 834 were expected to represent some of theoldest Lau Basin crust, and paleontological dating of sediments over-lying the inferred volcanic basement implies that basin opening andvolcanism had begun by 5.6 Ma. The volcanics recovered includeplagioclase-olivine-clinopyroxene basalts, exhibiting varying degreesof fractionation with mg ratios (= mol% Mg/Mg + Fet) ranging from33 to 67, including relatively Mg-rich tholeiites (mg = 61-67), andhigh Fe-Ti basaltic andesites (mg = 33-43). Site 835 lavas are overlainby late Pliocene sediments (ca. 3.5 Ma), and comprise relativelyunfractionated tholeiites (mg = 55-63). Site 836 volcanic basement(minimum age 0.7 Ma) consists of olivine tholeiites (mg = 61-68),basaltic andesite (mg = 45-^7) and andesite (mg = 36-39). Sites 837and 838 (ca. 2.0 Ma minimum ages) yielded only limited volcanichorizons. These comprised some 4 m of two-pyroxene andesite (mg =34-35) (Site 837), and gravels of basaltic andesite (mg = 40-50) andandesite (mg = 28) (Site 838). Site 839, with a probable age of 1.25 to>2.2 Ma, comprises a sequence of remarkable olivine-phyric, high-MgO tholeiites to picritic basalts (mg = 57-75), together with magne-sian tholeiites (mg = 64-67) and two-pyroxene basaltic andesites (mg= 39-50). Detailed stratigraphy of the units recovered at each site aregiven in Parson, Hawkins, Allan, et al. (1992).

385

A. EWART, W.B. BRYAN, B.W. CHAPPELL, R.L. RUDNICK

The modern volcanoes of the Tongan Islands are dominated bytwo- pyroxene + Plagioclase basaltic andesites with minor andesitesand dacites, the most notable of the latter being on the island ofFonualei (Ewart et al., 1973). Even more remarkable is the 1969Metis Shoal eruption (Melson et al., 1970) of low K2O rhyolitic glasswith xenocrysts of olivine (Fo93), bronzite, diopsidic augite, andbytownite. The Kermadec Islands are more diverse, with significanteruptions of olivine basalts, together with two-pyroxene and plagio-clase basaltic andesites, andesites, dacites, and rare low-K rhyolites(Ewart et al., 1977).

METHODS AND TECHNIQUES

The approach taken has been to use existing published literature tocharacterize the gross geochemical features of the currently recognizedmajor spreading ridge systems, namely, the Central Lau SpreadingCenter (CLSC), the Intermediate and Eastern Lau spreading centers(ILSC and ELSC), the Valu Fa Ridge (VF), and the King's TripleJunction (KTJ) in the northeastern part of the basin (Fig. 1). The firstfour of these divisions follow from the imagery results of Parson et al.(1990), together with the inferred history of development of theseridges as southward-moving propagators (Parson and Hawkins, thisvolume), an interpretation with considerable geochemical implica-tions. The KTJ (Falloon et al., 1992) is inferred to represent threeintersecting spreading centers and a transform fault (Parson et al.,1990), situated in the northeastern Lau Basin. As far as possible, onlydata from these ridge systems are included, with seamounts beingspecifically excluded from this compilation. The omission of seamountdata is justified on the basis that these may represent superimposed,localized thermal plume features, and thus are not necessarily directlyrepresentative of the actual propagating ridge systems. Nevertheless,some comparative data from the subaerially exposed active volcano ofNiuafo'ou (Fig. 1) are included for comparison.

In terms of interrelating the history of volcanism of the Lau Basin,as seen in the Leg 135 cores, with the later development of the ridgepropagator systems, it is geochemically critical to note the positionsof Sites 834 to 839 with respect to the two morphotectonic domainsrecognized by Parson and Hawkins (this volume). These comprise thewestern half of the basin in which a horst-and-graben topography isdeveloped, and the younger (~<2 Ma) eastern half is characterized bymore typical seafloor spreading fabric. The latter region is interpretedas having formed from the still active propagating spreading ridges.Sites 834, 835, and 838 lie within the western region, and Site 839 isvery close to the boundary, but just inside the western region, whereasSite 836 and possibly Site 837 (Hergt and Hawkesworth, this volume)lie within the eastern zone, presumably on ELSC crust. It is relevant,therefore, to note the importance of especially Site 836 in the follow-ing interpretations.

In the mineralogical, major element, and trace element plotspresented in this chapter, the following data sources are used: Bauer(1970), Baker et al. (1971), Oversby and Ewart (1972), Ewart et al.(1973, 1977), Ewart (1976), Gill (1976), Hawkins (1976), Johnstone(1978), Kay and Hubbard (1978), Hawke (1983), Cole et al. (1985,1990), Hawkins and Melchior (1985), Valuer et al. (1985, 1991),Ewart and Hawkesworth (1987), Falloon et al. (1987, 1989, 1992),Jenner et al. (1987), Frenzel et al. (1990), Boespflug et al. (1990),Loock et al. (1990), Sunkel (1990), Falloon and Crawford (1991), andErnewein et al. (in press). Additional unpublished Kermadec datawere made available by Dr. J.A. Gamble. Extensive use of the Leg135 shipboard X-ray fluorescence (XRF) data is made and includedin all relevant element plots. These are supplemented by new modaldata (Table 1) and new trace and major element analyses of 38samples, 26 from Sites 834-839 and the remainder from the Tongaand Kermadec Islands (Tables 2-5).

Trace element analyses of oceanic island arc related lavas areespecially challenging because of the very low abundance levels ofthe high-field-strength elements (HFSEs), the rare-earth elements

(REEs), and even the LILEs (e.g., Rb, Cs), even though the latter arerelatively "enriched." As is evident from the data presented in this andrelated papers in this volume, the Lau-Tonga lavas exhibit moreextreme depletions of most of these elements than other documentedarc systems. To check the general validity of the trace element anal-yses, all samples were analyzed by XRF (University of Queensland),inductively coupled plasma mass spectrometry (ICP-MS) (MonashUniversity), and instrumental neutron activation analyses (INAA)(Australian National University). In addition, six samples have beenanalyzed by spark source mass spectrometry (SSMS) (AustralianNational University). The separate data sets are shown in Tables 2-5.Although differences are apparent among the techniques for someREE, LILE, and HFSE data, on the broad comparative data plots beingused (e.g., the spidergrams), the data form encouragingly coherentdata sets. A discussion of the comparative data obtained by the varioustechniques is presented in the Appendix. For purposes of internal con-sistency, however, only the XRF and ICP-MS data are used in theaccompanying elemental plots. Details of analytical techniques usedare provided in the Appendix.

MINERALOGY

Composite plots of Plagioclase, olivine, and pyroxene composi-tions are presented (Figs. 2—4) for the major units from Sites 834 to837, noting that the phenocrystal assemblage Plagioclase ± olivine ±augite (+ orthopyroxene in basaltic andesites and andesites of Site837) occurs throughout these sequences. Comparable mineral data forthe Site 839 lavas are presented in Ewart et al. (this volume), whereasdetailed documentation of the oxide phases are presented by Allanand also Nilsson (both in this volume).

Plagioclase phenocryst compositions in Site 834 lavas are domi-nantly bytownite, being most calcic in the most magnesian Units 5and 7 (An77_89), and least calcic in the relatively fractionated Unit 12(An51_58). Within Site 835, compositions are bytownite, but morevariable (An72_90, predominantly between An73_77), probably in partreflecting the gradation from phenocryst to microphenocrysts. Plagio-clase compositions within Site 836 lavas are similarly bytownite(An82_9o), being predictably most calcic in the relatively MgO-richUnit 3 (mg 66), and least calcic in the andesitic Unit 1. Even withinthis latter unit, however, core compositions of >An89 sporadicallyoccur, suggesting the preservation of xenocrystal (or inherited) pla-gioclase remnants from more primitive parental melts. Within Site839, plagioclases are bytownite within the tholeiitic and picritic Units1, 3, 4, and 6, but are more calcic in the basaltic andesites of Units 2and 9, extending into the anorthite range. In this respect, the Site 839basaltic andesites are similar to the modern Tofua Arc basaltic an-desites (see fig. 2 in Ewart et al., this volume). Such highly calciccompositions in the basaltic andesites suggest "inherited" relict pla-gioclase from more primitive precursor magmas; even these, how-ever, would need to represent low sodium melts to precipitate suchcalcic plagioclases (e.g., Falloon et al., 1988).

Olivine phenocryst and microphenocryst compositions (Fig. 3)within Site 834 lavas extend up to Fo89 (Unit 7), to Fo63_71 for therelatively fractionated Unit 12. The majority of olivines range be-tween Fo78 and Fo88. Compositions within the Unit 3 tholeiites of Site836 are Fo86_89, being slightly lower in Unit 4 (Fo82_87), and more soin the basaltic andesite of Unit 5 (Fo74_79). Within Site 839, olivinesoccur in the Unit 1 tholeiite (Fo81_87), the picritic Units 3 and 6(Fo83_92, mostly between Fo90 and Fo92), and Unit 4 (Fo82_90).

One major point of significance concerns the extreme level ofmagnesium enrichment represented by the olivines in these variousbackarc lavas, with those of Units 3 and 6 in Site 839 being the mostmagnesian olivines encountered in the Leg 135 drill sites. Thesecompositions compare with reported compositions of Fo92^ for thenorthern Tongan boninitic lavas, Fo906 and Fo9M for the Valu Fa andELSC, Fo87 for KTJ, Fo86 for CLSC, and Fo83 for the Niuafo'ou lavas.Very forsteritic xenocrystal olivines (Fo90_93) are also reported from

386

REGIONAL GEOCHEMISTRY OF LAU-TONGA SYSTEMS

179

-15°

x PACIFICX , . PLATE

Figure 1. Location map of Lau Basin, showing bathymetry (km), locations of Lau and Tonga ridges, and Leg 135drill sites (including DSDP Site 203). Islands shown are Niuafo'ou (NF), Tafahi (Taf), Tongatapu (T), 'Eua (E), Ata(A), Upolu (U), and Vava'u (V). Also shown (dashed line) is the approximate boundary between the older westernand younger eastern Lau Basin provinces (Parson and Hawkins, this volume). CLSC = Central Lau Spreading Center,ELSC = East Lau Spreading Center, ILSC = Intermediate Lau Spreading Center, and KTJ = King's Triple Junction.

Metis Shoal and Tofua in the modern Tofua Arc. The significance ofsuch magnesian olivines lies in the implied refractory nature of theirmantle source (e.g., Duncan and Green, 1987). The most magnesian(or "refractory") compositions seem to occur in those magmas show-ing strongest arc-like geochemical signatures.

Pyroxenes (Fig. 4) in the Site 834 lavas range from diopsidicaugite to augite, with limited Fe-enrichment occurring in interstitialgroundmass grains. Slight Fe-enrichment also occurs in phenocrystalaugites in the relatively fractionated Unit 12. No trends towardsubcalcic and low-Ca compositions are evident. Very similar pyrox-ene compositions occur in the Unit 4 tholeiites of Site 836. The

tholeiites of Site 835 carry similar diopsidic augite phenocrysts, butslightly less calcic than those occurring in the Site 834 lavas. Incontrast, the groundmass pyroxenes occurring in the basaltic an-desites of Unit 5, Site 836, and the Site 837 andesites, exhibit trendsinto the subcalcic augite fields, whereas the Site 837 andesites alsocontain coexisting phenocrystal augite and orthopyroxene. Data forthe Site 839 pyroxenes (fig. 4 in Ewart et al., this volume) illustratethe tendency for the phenocryst and microphenocryst pyroxenes fromthe tholeiitic and picritic Units 3 and 4, and the basaltic andesites ofUnits 2 and 9, to extend to less calcic compositions; the data also showthe extensive development of subcalcic to pigeonitic compositions in

387


Table 1. Summary of modal data (vol%) for newly analyzed samples, Sites 834-839.

Hole:Core, section:Interval (cm):Unit or subunit:

834B8R-212-18

2

834B11R-386-89

5

834B15R-247-56

6

834B31R-373-78

7

834B35R-158-60

8

834B37R-136-42

9B

834B40R-157-6010A

834B55R-123-25

12

834B57R-1

126-12813

835B4R-1

131-1411

835B7R-115-22

1

836A3H-330-90

1

836A4H-CC0-13

3

836B6R-1

105-1104B

836B9M-1

117-1225

Phenocrysts:PlagioclaseClinopyroxeneOlivineOrthopyroxeneCr-spinelMagnetiteTotal phenocrysts

Groundmass:PlagioclaseClinopyroxeneOrthopyroxeneOlivineOpaquesMesostasisVesiclesVesicle infilling

N

0.2—

———

0.2

42.726.7

——

1.828.57.15.6

1276

——

————

49.4C

26.0c

——

1.822.85.36.2

1400

9.34.11.3

———

14.7

2.90.8

——

81.721.3

—

1640

11.1—

0.4———

11.5

—

—<O.l

88.5

—

1725

8.0a

5.2"1.6a

———

14.8

2.2———

92.918.0—

1447

13.41.61.9—

—16.9

31.821.5

—0.2

29.710.5

—

1033

0.8"0.7"1.4"———

2.9

12.9——

94.323.1

—

1102

8.5"2.62.0———

13.1

5.1

——

0.381.513.9

—

1723

9.02.80.9———

12.7

1.50.5—

85.315.7—

1613

11.6'5.7a

3.1"———

20.4

1.2————

78.58.4—

1931

11.30.9'2.6———

14.8

12.19.9e

———

63.214.3

—

1427

——

———

0

5.03.0———

92.030.1

—

1073

13.7a

—3.5"

———

17.2

—————

82.9

—

1721

————

<<O.l—0

47.0d

27.2d

—4.0d

1.020.711.65.8

1340

17.1a

0.10.4———

17.6

7.421.9

——

53.017.5—

1500

Notes: Samples have been recalculated vesicle-free. N = number of points counted.a Gradation of Plagioclase, clinopyroxene, ± olivine from phenocrystal to microphenocrystal size ranges.

Phenocrystal to microphenocrystan size, often with clinopyroxene rims.c Plagioclase seriate textured, mostly relatively coarse groundmass; pyroxene ophitic.

Seriate texture.e Seriate textured from microphenocrystal to groundmass size, often in aggregates.

Coarse groundmass to microphenocrystal size, also as inclusions in clinopyroxene.

the Unit 1 tholeiite, and the groundmass pyroxenes within Units 2 and9. In addition, these latter two units carry phenocrystal bronzite. Thecompositions of the pyroxenes in the Site 839 basaltic andesites are,in fact, closely similar to those occurring in the modern Tofua Arcbasaltic andesites (fig. 4 in Ewart et al., this volume). Relativelylimited pyroxene data are available for the CLSC, ELSC, and Valu Falavas (Figs. 4J-4L). These indicate extensive development of subcal-cic to low calcium pyroxenes in the ELSC and Valu Fa lavas.

One aspect, therefore, shown by the above mineralogical compari-sons is the closer affinity shown by the Site 839 magmas to those ofthe ELSC, Valu Fa, and especially modern Tofua Arc lavas. A secondimportant aspect concerns the possible role of mixing within the Leg135 lavas, as shown, for example, by the isotopic and trace elementdata for Site 834 (Hergt and Nilsson, this volume) and the Cr-spineldata for Units 1 and 3 at Site 839 (Allan, this volume). To investigatepossible mineralogical disequilibrium, olivine compositions are plot-ted (Fig. 5) vs. whole-rock Mg ratios (atomic % Mg/Mg + Fe2+, whereFe2O3/FeO is assumed equal to 0.2). Three sets of Fe-Mg partitioncoefficient ratio curves are plotted for comparison, which encompassthe range of plausible "equilibrium" values that may be expected. Theresults suggest "nonequilibrium" compositions of olivines in Units 1and 3 from Site 839, also supported by the data for the singlematrix-olivine pair from Unit 3. Figure 5 also suggests, however, thatcertain of the sequences within Sites 836 (Unit 4) and 834 (Units 6,7, and 9) exhibit olivine compositions not in equilibrium with whole-rock values. These data, therefore, certainly lend support to the roleof mixing processes in the petrogenesis of certain of the Leg 135 lavas,although the olivine data taken alone do not necessarily preclude suchprocesses as the accumulation of crystals or the occurrence of xeno-crystal olivines.

MAJOR ELEMENT DATA

Figures 6A-6G illustrate the normative compositions of the Sites834 to 839 lavas, together with data from the Lau spreading centers,Tonga Arc, and Kermadec Arc, projected from the Plagioclase ontothe olivine-clinopyroxene-silica plane (after the method of Grove etal., 1982). The inferred phase boundaries are after Grove and Bryan

(1983) and Grove et al. (1982). Comparisons of data in these plotssuggest the following points:

1. The lavas from Sites 834 to 838 project either close to, or on,the olivine side of the 1-bar cotectic, consistent with the observedphenocryst assemblages of Plagioclase + olivine ± augite. The con-sistency with which the data is displaced toward the olivine side ofthe saturation surface is suggestive of crystallization under significantwater pressure (see Gaetani et al., this volume). The most silica-richsamples (including all analyzed samples from Sites 837 to 838) areorthopyroxene-bearing, consistent with their projected compositionsrelative to the orthopyroxene-saturated surface(s). The bulk of thedata plot in the silica-saturated field; additional data from glasses andwhole rocks (Bryan et al., this volume) also indicate that silica-undersaturated compositions do occur in Site 836.

2. The general compositional fields observed for Sites 834-836compare closely with the ELSC, CLSC, and KTJ lavas, although thelavas from VF, and to a less extent the ELSC, show a greater pre-dominance toward more silica-enriched compositions, and in thisregard may be compared with the modern Tongan and Kermadec axialarc volcanoes. Most of the data exhibit an apparent displacementtoward the inferred 1-Kbar water pressure olivine-pyroxene satura-tion surfaces (Grove et al., 1982).

3. Site 839 is unique with respect to the occurrence of olivine-phyric, high-MgO lavas (Units 3, 4, and 6), together with magnesiantholeiite (Unit 1) and basaltic andesites (Units 2, 5, 7, and 9). Petro-graphic data indicate that Unit 1 lavas are at or close to Plagioclasesaturation, whereas the bulk compositions of Units 3 and 6 are not,consistent with their projection well within the olivine field. Plagio-clase, however, occurs in the groundmass of these lavas, consistentwith the projected composition of the separated matrix from one ofthese lavas (tie-line in Fig. 6B). The basaltic andesites project withinthe orthopyroxene field, consistent with the occurrence of Plagioclase+ orthopyroxene + augite phenocryst assemblages. The projectedcompositions of Units 1 and 2, 5, 7, and 9, which are considered tomost closely define the olivine-pyroxene saturation surfaces, areagain clearly displaced toward the 1-Kbar water pressure surface,consistent with the experimental data of Gaetani et al. (this volume).


Table 1 (continued).


Phenocrysts:PlagioclaseClinopyroxenesOlivineOrthopyroxeneCr-spinelMagnetiteTotal phenocrysts

Groundmass:PlagioclaseClinopyroxeneOrthopyroxeneOlivineOpaquesMesotasisVesiclesVesicle infilling

N

837B3R-1

141-1451

0.92.7—

0.2b

——

3.8

23.412.4

——

0.2r

60.113.6

—

1245 :

837B5R-11-81

1.61.1—

0.1b

——

2.8

18.410.6

——

0.2f

67.911.2

—

1346

838A20H-170-114

839B13R-225-30

(volcanic gravel) 1

4.10.5

0.3—0.55.4

——

94.6——

1497

0.95.1a

2.9a

—0.1

—9.0

30.015.3

—

45.514.3

—

1221

839B20R-129-35

3

—4.8a

10.9a

—0.2

—15.9

8.111.7

—

64.236.0

—

1339

839B23R-10-6

3

—1.1"

16.2a

—0.2

—17.5

<O.l19.6

—0.8

62.134.1

—

2066

839B25R-127-32

3

—1.2

14.7a

—0.2

—16.1

5.410.9

———

67.733.2

—

1867

839B25R-133-41

3

—0.3

27.8—

0.3—

28.4

1.410.3

———

60.020.1

—

1252

839B30R-

79-8419

10.50.6

—0.6

——

11.7

15.310.1

——

0.162.814.5

—

1328

839B35R-17-11

9

14.80.1

—0.7

——

15.6

11.5————

73.017.7

—

1332

839B38R-18-12

9

9.40.4

—0.3

——

10.1

12.52.9

———

74.64.8

—

1315

It is relevant to compare the high-MgO basalt data from Site 839 withthe boninites of northern Tonga (Fig. 6B). Both sets of compositionsproject into the olivine field at 1 arm., with fractionation of olivineand minor pyroxene evidently controlling compositions toward theolivine + augite + Plagioclase saturation surfaces, and thus exhibitcomparable major element behavior. The boninites, however, areclearly displaced toward more silica-enriched compositions, even atequivalent projected olivine compositions.

The previous observations are reinforced by considering the datafields in terms of the projection from clinopyroxene onto the plagio-clase-olivine-silica surface (Figs. 7A-7D). The fields for the Leg 135sites (except Site 839) exhibit complete overlap with the total datafield for the Tonga-Kermadec volcanoes. In contrast, the fields for themodern Lau spreading centers (Fig. 7B) exhibit greater variability,possibly pressure controlled. Again, however, the compositional fieldsand trends of the boninites and the Site 839 lavas are distinct.

Further comparisons between the various suites can be made (Figs.8-11) by means of variation diagrams, using MgO as the majorvariable, with Figure 8 showing the overall MgO distributions withinthe main subregions of the Lau-Tonga-Kermadec region. These dataillustrate the overlapping range in MgO from Sites 834 to 836, withthe tholeiites from the latter site extending to slightly higher valuesthan in the lavas from Sites 834 and 835. Lavas from the CLSC, KTJ,and ELSC-ILSC exhibit similar although more variable MgO, whichextend to both lower and higher MgO than Sites 834 to 836. The ValuFa lavas, however, are represented by a higher proportion of lowerMgO and are more fractionated compositions, therefore, consistentwith previously presented chemical and mineralogical data.

The Site 839 lavas are again distinctive in terms of MgO, mostnotably the picritic units, which invite obvious comparison with thenorthern Tongan boninitic series. The arc-related lavas of the Tonga,Kermadec, and Lau islands all exhibit a similar tendency for a higherproportion of relatively low-MgO types compared to the Lau Basinspreading centers, with the notably exception of the Valu Fa lavas(which exhibit strong arc-like geochemical signatures).

Of the other major elements, variation diagrams of MgO vs. CaO,Na2O, and TiO2 are specifically illustrated. These plots emphasize therole of fractional crystallization processes in the evolution of thevarious magma systems. The CaO-MgO relations are chosen as theyillustrate well the distinctiveness of the boninites and the Site 839MgO-rich lava suite. These clearly show increasing CaO with de-creasing MgO, reversing trends only when MgO levels have droppedto near 7%. This is suggestive of fractionation dominated by olivine ±

orthopyroxene. The other volcanic suites exhibit decreasing CaO andMgO, although in the cases of the ELSC and ILSC data, indicationsare that this trend flattens above about 9% MgO.

Of the major elements, Na2O and TiO2 (Figs. 10-11) provide themost distinctive discrimination between the magmatic groups of theregion, with both Na and Ti exhibiting negative correlations withMgO. At low MgO contents, TiO2 shows a rapid depletion in someof the volcanic suites (presumably related to Fe-Ti oxide precipita-tion). Considering first the Leg 135 site data, it is evident that the Site834 lavas are, for given MgO contents, the most Na- and Ti-enriched,whereas the Site 839 lavas are the most depleted. Site 835 lavasoverlap those of Site 834, whereas lavas from Sites 836 and 837 areintermediate. The very limited numbered samples from Site 838suggest that they may extend the trend shown by the Site 839 samples.Although discussion of these differences will be given subsequentlyin this paper, it is relevant to note here that the differences are inferredto be source related.

Comparison of the Leg 135 data with the modern arc and spread-ing ridge lavas also reveals some interesting comparisons. For exam-ple, although overlap occurs, a pattern is present of increasing TiO2,at given MgO levels, passing from the VF, through the ELSC, to theCLSC. A similar, but less clearly defined pattern is apparent for Na2O.For both elements, the Site 834 data show the closest overlap withthe CLSC and the KTJ. Comparison of Site 839 with the Tongaand Kermadec lavas suggests overlapping abundances of Na2O, al-though the absence of high-MgO lavas in the modern axial arc vol-canoes precludes a more complete comparison. TiO2 is similar be-tween the Kermadec and Site 839 lavas, but the lavas from this lattersite are mostly slightly enriched in TiO2 (at given MgO abundances)compared with the majority of Tongan lavas. The northern Tonganboninites are more TiO2 depleted than the Kermadec and Site 839lavas, although they overlap with respect to Na2O in the Site 839 andmodern Tongan lavas.

In summary, the major element data show that apparently continu-ous spatial and, presumably also, temporal variations of chemistryexist within the lavas of the Lau Basin. Moreover, a set of near-parallelvariations are recognizable in the lavas intersected at Sites 834-839.The basaltic andesites of Site 839 have a distinctly arc-like chemistry,whereas the Site 839 high-MgO lavas can perhaps be envisaged astrending toward boninites in their overall chemistry. In contrast, theSite 834 lavas seem to be similar to those of the CLSC, with chemis-tries close to N-MORB (e.g., see also Loock et al., 1990; Sunkel,1990; and following discussion). Site 836 glasses also show overlapwith ELSC chemistry, although data are limited for Site 836.

3X9


Table 2. New major and trace element analyses of lavas from Site 834.


834B8R-212-18

2

834B11R-386-89

5

834B13R-1

130-1315

834B15R-247-56

6

834B31R-373-78

7

834B33R-2

105-1107

834B35R-158-60

8

834B37R-136^2

9B

Major elements (wt%):SiO2TiO2

A12O3

FeOMnOMgOCaONa,0K2OP2O5TotalLOI

51.511.41

16.263.635.880.137.11

10.963.190.240.14

100.-

Trace elements (ppm):

CsRbSrZrNbYUThPbNiCrVSeCoCuZnGaBaLaCePrNdSmEuGdTbDyHoErTmYbLuTaHfSbWTIBeLi

a0.031.35

17199.4

1.4525

0.120.181.47

4065

2763439637619.536.1

3.9211.31.99.653.191.144.060.744.651.032.890.422.520.390.102.38

0.190.420.20

11.6

4.13

b<O.l

59

3838

353.6

10.8

9.02.81.163.80.7

1.0

2.60.39

1.8<O.l

50.481.09

17.013.395.170.138.35

11.242.840.190.11

100.

a0.030.63

15482.5

2.4821.3

0.060.180.66

92246213

3538656818.5

2583.049.301.667.992.681.063.480.714.190.942.770.422.380.420.082.03

0.030.260.19

12.0

5.45

b<O.l

210

3237

352.57.9

6.82.20.923.00.5

0.7

2.10.31

1.4<O.l

49.971.18

16.843.794.910.158.82

11.552.890.130.10

100.334.90

a

1.216480.7

1.221.9

1.895

2022103035687117.337

0.217.3

a0.124.5

165147

1.8937.1

0.120.211.16

431162994339549620.526.6

4.6814.82.66

13.324.561.575.931.256.901.584.430.714.330.700.123.64

0.071.290.246.95

51.901.80

15.723.726.170.185.83

10.483.520.390.20

99.911.81

b<O.l

96

3537

354.3

14

12.43.81.505.20.9

1.5

3.70.54

2.7

a0.854.5

16997.5

1.6023.5

0.130.070.83

1013712224139647219.96.53.159.951.668.402.750.983.680.644.210.922.600.352.210.330.102.31

0.080.260.22

22.9

50.161.29

17.804.373.930.156.00

12.583.010.21.0.12

99.622.52

b0.45

320

3338

202.99.4

8.12.61.103.70.6

1.0

2.50.37

1.60.90

49.321.27

17.773.804.530.138.17

11.803.190.070.10

100.153.16

a

0.4156

44.81.6

23.7

883131873134566419.910

52.151.15

16.422.486.040.157.02

11.762.630.260.11

100.17

a0.142.3

14881.1

1.1023.5

0.090.131.00

552092494339567918.832.0

2.488.231.457.612.650.943.650.684.200.932.660.362.320.360.082.62

0.050.950.175.61

1.60

b<O.l

190

3738

352.27.2

6.92.41.023.50.6

0.9

2.50.37

1.5<O.l

51.051.62

16.023.905.960.176.66

11.112.990.370.16

100.

a0.366.4

132119

1.3534.5

0.060.100.86

82209266

3843538721.1

8.603.19

11.11.99

10.643.801.344.950.906.011.333.760.533.390.540.102.89

0.061.000.214.92

1.73

b0.20

190

3541

203.0

10.6

9.93.31.364.70.8

1.3

3.40.51

2.31.5

Notes: Major element values (wt%) were determined by X-ray fluorescence, with Na2O determined by atomic absorption. Loss of ignition (LOI) corrected for oxidation of Fe. Traceelements based on the following techniques: Column a: ICP-MS (D. Lambert and W.W. Ahlers, Monash University); X-ray fluorescence (F. Audsley, University of Queensland,specifically Rb, Sr, Zr, Nb (in part), Y, Ni, Cr, V, Se, Co, Cu, Zn, and Ga); and atomic absorption with graphite furnace (Be, Pb) and flame photometry (Li in Tables 3 and 5) (R.Hall, University of Queensland). Column b: INAA (B.W. Chappell, Australian National University). Column c: Spark source mass spectrography (R. Rudmick, Research School ofEarth Sciences, Australian National University). All trace element values in parts per million (ppm).

TRACE ELEMENT DATA

Regional Comparisons

The general abundance patterns of the compatible and incompatibleelements are compared in Figures 12-14. The Ba and Zr-MgO plots(Figs. 12-13) illustrate the incompatible element patterns, with bothelements increasing with decreasing MgO, a trend certainly reflectingthe overall control by crystal fractionation processes throughout thevarious subprovinces of the Lau-Tonga system. The differing rates andlevels of Ba and Zr increase, however, clearly reflect the abundancesof these elements in their parental melts. Comparing Site 834 to Site839, it is evident that the Ba variation fields for Sites 834-836 overlap,with the higher MgO (>6%) samples having Ba levels lower than 60

ppm. The lavas from Sites 837 to 839 are characterized, in contrast,by higher Ba abundances, and at the low MgO end of their composi-tional spectrum, fractionate to abundances in excess of 100 ppm. Thereverse is observed with Zr, in which Site 834 lavas clearly are Zrenriched at given MgO abundances, compared with the lavas of othersites, and also fractionate to relatively higher levels. The Site 839sequence lies at the lower end of the Zr abundance spectrum. Thus,as with the major element data, evidence can be found for systematicdifferences in incompatible trace element patterns among the lavasfrom the six Lau Basin drill sites.

The trace element geochemical patterns for Sites 834-839 arecompared with the overall Lau-Tonga arc and modern spreadingcenters in Figures 12 and 13. It is evident that, with respect to Ba, themodern arc lavas of Tonga and Kermadec are relatively enriched, and

390




Major elements (wt%):SiO2

TiO2

A12O3

Fe2O3

FeOMnOMgOCaONa2OK2OP 2 O 5

TotalLOI


CsRbSrZrKbYUThPbNiCrVSeCoCuZnGaBaLaCePrNdSmEuGdTbDyHoErTmYbLuTaHfSbWTIBeLi

;334B40R-157-60

10A

52.41.66

16.55.725.860.184.579.413.370.690.19

100.7

a0.308.2

175110

1.2131.30.150.271.78

2113

352333729

10020.148.2

3.9412.052.10

10.933.801.314.860.865.701.263.570.503.210.510.122.83

0.092.580.208.24

4.71

b0.15

7

2936

453.6

11.0

9.83.21.324.40.7

1.3

3.30.48

2.10.10

834B55R-123-25

12

51.972.16

15.686.076.950.224.028.313.840.610.22

100.053.22

a b0.21 <O.l9.1

180143

1.5538.40.090.141.446.57 3

41333 3140 4031

12624.446.3 55

4.6214.65 13.82.58

13.3 13.04.44 4.11.61 1.655.94 5.51.04 0.96.901.54 1.54.350.603.93 4.10.62 0.610.133.68 2.8

<O.l0.134.110.216.49

834B57R-1

126-12813

51.411.19

16.682.895.920.167.24

11.912.700.190.11

100.401.5

a b0.09 <O.l0.94

15481.80.91

23.20.080.281.24

61217 20025543 3839 37578016.628.8 35

2.85 2.58.55 8.11.718.3 7.53.10 2.61.23 1.084.00 3.60.86 0.74.841.14 1.03.240.552.67 2.60.55 0.390.072.01 1.5

<O.l0.070.750.234.09

fractionate (at lowest MgO) to concentrations in excess of 300 ppm.Interestingly, there are some systematic differences between the south-ernmost Tonga arc volcano of 'Ata and the remaining Tongan lavas,with the 'Ata lavas being more Ba enriched at given MgO levels. Suchdifferences extend to other trace elements. Moreover, a significantoverlap is evident between the modern Tonga and the Site 839 Ba-MgOdata. Within the Lau Basin itself, it is evident that Valu Fa and KTJlavas are Ba enriched compared to the CLSC and ELSC lavas.

Reference to the Zr-MgO data reveals complementary comparisons,with the modern arc lavas relatively Zr depleted, and the CLSC andELSC lava relatively Zr enriched. The Valu Fa and King's Triple Junc-tion are intermediate in their abundance patterns, overlapping those ofSites 835-838. Overlap occurs between Site 834 and CLSC lavas,whereas Sites 835-838 overlap with the ELSC and Valu Fa fields.

Finally, it is relevant to compare the incompatible element patternsbetween the Tongan axial volcanics, the north Tongan boninites, andthe Site 839 olivine-phyric, highly magnesian lavas (most especiallyUnit 3). In terms of Zr, the two groups of Tonga lavas are closely com-parable; however, with the boninites, they are more enriched in Ba.

Compared with Site 839, the boninites are depleted in Zr (althoughwith some slight overlap) but much more enriched in Ba.

In summary, it is evident that the abundance patterns for Zr and Ba,although clearly affected by fractional crystallization processes, arenevertheless distinct within the various subprovinces of the arc-back-arc system. In particular, the extremes of the compositional spectraoccur in the CLSC at the one end, and the modern arc lavas at the otherend, with the ELSC, Valu Fa, and King's Triple junction lavas lying atintermediate points. Significantly, parallel variations are seen withinthe Sites 834-839 data, with Site 834 lavas similar to the CLSC lavas,and Site 839 lavas showing closest similarities to the modern arc lavasand the north Tonga boninites. These differences are consistent withthe regional variations shown by especially Na2O and TiO2.

Compatible element behavior (Fig. 14) is illustrated by Ni andserves to emphasize the effects of fractionation. Thus, Ni ranges fromrelatively high concentrations (>300 ppm) to depleted abundances inthe most fractionated magmas, all suites exhibiting continuous rangesof variation. The Ni abundances correlate with MgO, the highest abun-dances occurring in the more abundant magnesian lavas of the CLSCand ELSC, the boninites, and Site 839 lavas of Unit 3. The Ni-MgOfractionation trends for these lavas follow similar trajectories that areclearly olivine dominated. The generally less magnesian lavas of theactive arc volcanoes and Valu Fa, however, seem to represent theterminations of somewhat shallower Ni-MgO fractionation trends,possibly indicative of the more important role of clinopyroxene dur-ing fractionation. If so, this implies higher pressure fractionation incomparison with the CLSC and ELSC magmas, but the possibilitythat these could also represent mixing trends cannot be excluded.

Although not shown, covariance between TiO2 and V exists andseems to reflect the control of both the concentration and depletion bythe specific stages of Fe-Ti oxide precipitation. In the Lau-Tonga lavas,the oxides evidently precipitate at relatively advanced stages of thefractionation processes, with the basaltic and basaltic andesites typi-cally being free of phenocrystal and microphenocrystal oxide phases,excepting Cr-spinel.

Composite Trace Element Comparisons

The previous major and trace data serve to emphasize the system-atic regional patterns of geochemical variation, both within the backarcbasin, and also within the arc volcanoes themselves. In Figures 15-19,therefore, a series of spidergrams are shown enabling more generalgeochemical comparisons to be made between the lavas of Sites834-839, the Lau Basin spreading centers (plus the subaerial OceanIsland Basalt (OIB) volcano of Niuafo'ou), the modern Tongan arcvolcanoes, the north Tonga boninites, the Kermadec axial volcanoes,and the Lau islands. The spidergrams have been compiled for all dataavailable for each subprovince considered, and are normalized to theN-MORB values of Sun and McDonough (1989). Also shown are theaveraged normalized values for N-MORB given by Hofmann (1988),these being based on a subset of slightly less magnesian samples(7.58% MgO) compared to the Sun and McDonough values, and theminimum Nl-MORB values of Viereck et al. (1989). These three setsof values thus give some indication of the range of variation of typicalN-MORB abundance patterns, an important factor when comparingother data sets with average MORB data. Data sets illustrated in thevarious spidergrams have utilized the more Mg-rich samples from eachsubprovince to attempt to minimize superimposed fractionation effectssuch that no samples with < 4% MgO are included. Although not ahighly stringent filter, the relatively fractionated chemistries of the arclavas make a higher level of MgO discrimination impractical. Thiscriterion, however, excludes data from Sites 837 and 838.

Comparison of the Leg 135 site data (Fig. 15) further suggestsprogressive changes from the Site 834 lavas, which are relatively"N-MORB-like," through Sites 836 and 835 and 839, the lattersequences exhibiting the marked depletions of HFSEs, together withLILE enrichments characteristic of arc lavas. Even the Site 834 data,

391


Table 3. New major and trace element analyses of lavas from Sites 835 to 838.



TiO2

A12O3

Fe2O3

FeOMnOMgOCaONapK2OP 2 O 5TotalLOI


CsRbSrZrNbYUThPbNiCrVSeCoCuZnGaBaLaCePrNdSmEuGdTbDyHoErTmYbLuTaHf"Sb

wTIBeLi

a0.092.4

12744.4

0.5221.80.060.110.72

64192278

4443688119.527.62.005.150.945.071.970.762.690.493.270.712.010.301.970.290.031.12

0.040.280.196.3

835B4R-1

134-1411

50.450.96

16.133.116.360.167.66

12.392.130.230.08

99.661.86

b<O.l

180

3941

352.15.2

4.51.90.843.10.6

0.8

2.40.36

1.00.15

835B7R-115-22

1

50.180.93

16.463.326.110.167.85

12.662.210.200.08

100.162.52

a b0.11 <O.l2.6

12343.5

0.4921.5

0.080.180.77

65190 19027441 4043 42658220.826.1 30

2.36 1.96.07 5.11.166.5 4.72.50 1.90.97 0.843.55 2.90.67 0.64.350.92 0.82.690.402.68 2.50.43 0.370.041.24 1.0

0.100.050.190.166.46

835B7R-2

75-841

50.801.04

15.524.155.750.138.19

11.432.590.120.07

99.793.00

a

1.912347.7

0.721.9

1.058

108289

3336859720.846

0.228.1

836A3H-330-90

1

55.491.37

14.763.408.890.204.178.442.820.270.12

99.932.11

a b0.15 0.104.7

13270.8

1.0028.10.110.101.23

1415 13

42743 3738 37

10411220.063.7 502.91 2.79.05 7.71.709.41 7.23.27 2.61.19 1.094.27 4.20.80 0.75.051.14 1.13.250.473.10 3.30.50 0.500.071.96 1.6

<O.l0.090.570.166.02

836A3H-333^3

1

55.511.35

14.952.949.450.203.988.453.390.230.13

100.581.68

a

2.713671.6

1.630.4

1110

4314031

10911320.065

836A3H-4

88-1002

57.031.21

14.702.788.930.263.658.123.260.280.13

100.352.06

a

4.413476.9

2.132.5

1.4134

3894034

12011220.279

0.266.9

836A4H-CC0-13

3

49.490.81

16.362.006.650.169.09

13.432.030.070.05

100.141.15

a b0.01 <O.l1.8

13642.60.45

17.60.040.0050.93

109384 330251

50 4246 45866717.27.93 200.7 1.52.18 4.40.442.68 3.90.9 1.50.37 0.701.31 2.20.26 0.51.760.4 0.71.140.150.98 1.90.16 0.290.031.18 0.8

<O.l0.040.20.174.6

836B5R-2

65-744B

49.450.83

15.953.585.280.159.59

13.512.170.050.04

100.603.21

a

1.013639.50.95

16.6

0.9495

254248

3041

1086917.019

0.195.6

836B6R-1

105-1104B

49.130.71

16.802.905.400.149.68

13.112.100.050.04

100.054.21

a b0.01 <O.l1.2

14036.90.41

14.90.050.0051.39

116371 34021840 4145 43795316.77.99 101.01 1.33.24 3.70.633.61 2.71.24 1.30.52 0.601.77 1.90.34 0.42.190.49 0.51.40.191.23 1.60.2 0.250.031.00 0.6

<O.l0.020.220.178.7

Note: See Table 2 for explanation of terms.

however, do contain slightly enriched alkali, Ba, Sr, and Pb abun-dances, and slightly depleted Nb and Ta compared to N-MORB. Thestatus of the Pb data is uncertain, possibly reflecting slight inconsis-tency in Pb analyses (cf. Appendix), whereas the higher alkali abun-dances, at least in part, reflect mild low-temperature alteration withinsome of the lavas (see Parson, Hawkins, Allan, et al., 1992). The over-all data for Site 834 are interpreted to be consistent with a chemistrysimilar to, but not exactly comparable with N-MORB. Hergt and Nils-son (this volume), in their detailed study of Site 834, show that sig-nificant trace element variation exists between the units, with Unit 7being in fact closest to N-MORB. The lavas from the other sites, how-ever, more clearly deviate from N-MORB, this being least for Sites835 and 836 and greatest for Site 839.

Comparison of the data for Sites 834-839 with the Lau Basinspreading centers (Fig. 16) again shows systematic differences andsimilarities. Thus, the CLSC and ELSC lavas are clearly closest toN-MORB in chemistry, but exhibit some alkali, and possibly Pb,enrichment, and a trend toward slight HFSE depletion; they are gen-erally comparable to Sites 834-836. The Valu Fa data, however, aredistinct and strongly arc-like. The KTJ lavas from northeast Lau are

similar to N-MORB for all elements except the alkalis, which also areenriched, whereas the Niuafo'ou lavas have similarly enhanced alkali,Ba, and Pb abundances. In many respects, however, this oceanicisland volcano is surprisingly MORB-like in its geochemistry (e.g.,see also Reay et al., 1974).

The comparative abundance patterns from Tonga (Fig. 17) showthe classic arc-type patterns, extending also to the north Tonga bonin-ites. It is evident, however, even from the qualitative comparison ofthe patterns, that differences exist in the relative depletion of theelements from Zr to Lu. Thus, the lavas from the northernmost volcano,Tafahi, are exceptionally depleted, similar to those of the more extremeboninite compositions. The lavas from 'Ata (southernmost Tongan vol-cano), Tofua, and Kao are the least depleted with respect to these sameelements. There is certainly an indication that the more northerly vol-canoes in the modern Tonga Arc are geochemically most stronglydepleted with respect to HFSEs and the middle- and heavy-rare-earthelements (MREEs and HREEs). Figure 18 compares available datafrom the Kermadec islands, which have been divided into three majorisland groups. As was recognized by Ewart and Hawkesworth (1987),the southernmost Kermadec Island of UEsperance is most MORB-

392



Hole:Core, section:Interval (cm):

836B9M-1

117-122Unit or subunit: 5

Major elements (wt%):SiO2TiO2

A12O3

Fe2O3FeOMnOMgOCaONa2OK2OP 2 O 5TotalLOI

52.921.21

15.663.048.420.205.56

10.712.510.340.10

100.671.76



a b0.05 <O.l4.6

13656.6

1.0623.9

0.090.011.45

2437 25

41243 4243 42919819.141.4 50

1.74 2.64.77 7.20.884.72 6.01.55 2.20.57 0.932.15 3.30.41 0.62.710.6 0.91.760.251.7 2.80.28 0.430.071.3 1.3

0.100.070.320.196.0

837B3R-1

141-1451

56.121.41

14.723.808.350.223.577.812.850.660.16

99.642.17

a b0.07 <O.l8.8

15174

1.8436.2

0.180.411.4629 8

33534 3433 3324

11520.479.9 60

4.79 3.812.9 10.82.31

12.1 9.44.15 3.11.48 1.245.59 4.61.06 0.86.751.43 1.24.260.664.43 3.80.73 0.580.152.57 1.8

0.100.080.40.184.4

837B5R-11-8

1

55.931.43

14.844.727.450.213.558.023.120.510.16

99.941.78

a b0.08 0.107.1

15775.5

1.9333.9

0.240.361.4517 8

33334 3332 3224

11522.282.5 654.9 3.9

13.3 10.42.35

12.3 8.74.26 3.01.51 1.245.78 4.51.08 0.86.881.46 1.24.360.664.52 3.80.74 0.570.182.66 1.7

<O.l0.080.330.195.6

838B20H-170-114

Volcanic gravel

63.651.11

13.632.406.740.191.905.393.860.570.27

99.712.80

a b0.17 0.356.2

157115

1.0541.6

0.190.211.88

<l4 3

7726 2618 1850

11220.6

160.2 1504.48 4.3

13.9 13.92.54

13.3 13.44.4 4.31.35 1.485.54 6.21.03 0.96.161.32 1.63.810.573.87 4.60.63 0.680.082.96 2.9

<O.l0.080.40.238.2

like, but the characteristic arc-like HFSE depletions are still present.The islands of the Raoul Group and Macauley are comparable to theTonga lavas in their overall geochemical abundance patterns, althoughthey demonstrate a relatively wide range of abundances. These aregreater than is readily explained by crystal fractionation alone (e.g.,Ewart and Hawkesworth, 1987) and is suggestive of more heterogene-ous magma sources than is characteristic of Tonga, an observation con-sistent with Pb-isotopic compositions (e.g., Oversby and Ewart, 1972).

The final spidergram comparison (Fig. 19) appropriate in thecontext of the broad regional geochemical patterns being consideredhere is for the Lau Islands, which form the western boundary to theLau-Tonga system. There are two distinct arc-like phases of volcanic-ity on these islands: the earlier widespread Lau Volcanic Group (LVG;14.0-5.4 Ma), and the more restricted Korobasaga Volcanic Group(KVG: 4.4-2.4 Ma) (Cole et al., 1985,1990). The youngest volcanics(Mago Volcanic Group; 2.0-0.3 Ma) comprise a more alkaline basalt-hawaiite association, geochemically and isotopically unrelated to theearlier volcanic phases, and not considered further in this report.

The spidergrams from the Lau and Korobasaga groups exhibittypical arc-like patterns, with extreme HFSE depletions that are more

clearly developed in the Korobasaga lavas. Compared to Tonga andthe northern Kermadec islands, however, the Lau and Korobasagavolcanics do not exhibit the same degrees of HFSE, MREE, andHREE depletion, an observation that applies also to comparisons withthe Site 839 and Valu Fa data.

Rare-earth Elements

In view of the evidence pointing to the existence of lava composi-tions extending from N-MORB-like to arc-like within the Lau Basin,it is relevant to compare the REE data for the basalts from the Leg 135sites with those of the modern Tonga-Kermadec lavas (Fig. 20).

The Site 834 REE patterns are clearly distinct from those of theremaining sites, being typically N-MORB-like with LREE depletion,and with abundances of MREEs to HREEs between 15× and 25×chondritic. In contrast, Site 839 data have clearly lower REE abun-dances (5×-12× chondritic) and flat to LREE-depleted patterns. Suchpatterns are comparable with those of the arc volcanoes, which varyfrom the very strongly depleted patterns of Tafahi (north Tonga) tothe near N-MORB-like patterns of UEsperance (southern Kermadecislands). Within the arc lavas, there is again a tendency for the morenortherly volcanoes in both the Tonga and Kermadec arc segments toexhibit the most depleted REE abundance patterns. In the case of theTonga Arc, the lavas from Kao are conspicuously the least depleted.

Within the Lau Basin Leg 135 sites, the lavas from Sites 835 and836 are also noteworthy for exhibiting LREE depletion and relativelylow total REE abundances, especially pronounced in Site 836. Thesefeatures are consistent, therefore, with the somewhat transitional spi-dergram patterns described above.

The REE data for the more silicic lava types are illustrated (Fig.20G), the generally relatively low abundances reflecting the depletedparental abundances, and the lack of LREE enrichment reflecting boththe parental LREE depletions and the phase assemblages involved inthe fractionation processes controlling the evolution of these magmas(Ewart and Hawkesworth, 1987).

DISCUSSION

Introduction

We have attempted to bring together all available chemical datafrom the modern Lau Basin spreading centers, the Lau Ridge, themodern Tonga and Kermadec arc volcanoes, and the north Tongaboninites. The primary reason for this synthesis is to put the Leg 135site data into the proper perspective for the overall regional geochem-istry. Perhaps one of the most critical observations is that the geochemi-cal variations and affinities of the lavas sampled at Sites 834-839 canbe equated to the lavas erupted either in the various Lau Basin spread-ing centers and/or by the modern arc volcanoes on the Tonga-Ker-madec Ridges. Moreover, the observed differences in the overallgeochemical variations observed across the Lau Basin, and along thearc ridges, are inferred to be characteristics inherited from source,although clearly the absolute abundances have been modified bysuperimposed crystal fractionation and mixing processes. Thus, theregional variations in chemistry imply a strongly heterogeneous Lau-Tonga-Kermadec upper mantle.

In a study of the petrogenesis of the Tonga-Kermadec magmas,Ewart and Hawkesworth (1987) identified two main factors thoughtto be responsible for the inferred heterogeneity of the mantle wedgebeneath the arc volcanoes.

1. The development of relatively refractory peridotitic sources asa result of prior depletion (or multistage depletion) by previousmagma extraction(s). These would be equivalent to the second andeven third stage melts of Duncan and Green (1987). Experimentaldata relevant to the nature of such melts is given by Falloon et al.(1988). The development of such refractory source(s) is thought to be

393


Table 4. New major and trace element analyses of lavas from Site 839.

Hole:Core, section:Interval (cm):Unit:

839B13R-225-30

1

839B20R-129-35

3

839B23R-1

0-63

839B25R-127-32

3 (matrix)

839B25R-133-41

3

839B30R-17-11

9

839B35R-17-11

9

839B38R-18-12

9

Major elements (wt%):SiO2TiO2A12O3

Fe2O3FeOMnOMgOCaONa2OK 2 O

P 2 O 5TotalLOI

52.780.68

15.333.335.730.168.81

11.541.680.270.08

100.:


CsRbSrZrNbYUThPbNiCrVSeCoQ i

ZnGaBaLaCePrNdSmEuGdTbDyHoErTmYbLuTaHfSbWTIBeLi

a0.39

1013730.7

0.3913.40.140.190.97

125820304

4744737915.451.6

1.975.180.874.741.740.692.340.482.700.631.770.311.700.320.050.82

0.070.440.166.65

1.31

b0.20

790

4442

551.74.5

3.51.40.592.00.4

0.6

1.70.26

0.2<O.l

50.80.60

13.93.695.530.16

13.7210.96

1.560.280.07

100.5

a0.094.5

13132.10.39

13.90.070.31.29

269

1.56

b<O.l

1237 1170247

4057707414.544.0

2.015.240.914.621.640.582.040.382.340.531.460.211.360.200.050.89

0.130.490.243.90

4054

452.05.1

3.41.40.591.80.4

0.5

1.40.22

0.6<O.l

a0.115

12431.6

0.2912.50.090.131.80

316

50.570.60

12.703.196.170.16

14.8310.64

1.420.290.07

100.642.07

b<O.l

c0.1055

139

1922 1730251

4158817416.342.6

1.704.570.804.201.460.541.840.342.160.481.360.201.250.190.020.80

0.041.010.164.32

3756

501.85.0

3.11.30.561.80.5

0.5

1.40.21

0.3<O.l

250.45

12.80.080.120.67

471.95.30.814.51.490.531.730.322.200.46

1.36

0.71

0.12

52.170.70

16.272.806.340.167.97

11.831.730.340.08

100.:

a0.144.9

166350.38

15.40.090.211.67

92607 :277

3839

1029316.649.8

2.536.141.125.682.010.702.410.452.690.611.620.251.540.230.02f0.98

(0.35)(2.60)0.224.99

1.71

b<O.l

540

3439

602.56.2

4.91.70.692.20.4

0.6

1.70.26

0.5<O.l

a0.053.7

10625.60.23

11.20.060.110.67

604

48.420.52

10.692.606.840.16

20.139.201.320.210.05

100.141.61

b<O.l

2396 2400212

3177607114.632.2

1.554.050.733.711.330.461.590.301.840.411.120.161.070.16O.Olf0.68

0.040.310.173.53

3575

451.63.8

2.51.20.501.70.3

0.4

1.20.19

0.2<O.l

c0.0692.9

9623.30.42

11.6

0.0750.55

331.64.50.703.741.230.431.380.251.770.3751.08

1.04

0.46

0.09

a0.07

10206

47.50.76

19.60.270.231.0489

3613933779319.9

1043.198.271.386.942.390.822.740.523.110.691.920.271.780.270.041.30

0.070.950.203.49

54.670.91

16.873.247.060.184.459.722.220.650.13

100.102.18

b c0.15 0.21

9.8218

450.65

210.160.201.55

7

3632

110 1133.3 3.658.6 9.46

1.46.8 6.832.1 2.340.86 0.832.8 2.550.5 0.48

3.340.7 0.70

2.06

2.2 2.110.32

1.1 1.06<O.l

0.13

a0.236.7

16940.7

0.3018.10.100.121.73

2032

3624731

9118.877.6

2.445.470.975.242.000.722.480.503.030.691.890.281.810.270.061.09

0.100.760.203.27

54.290.86

17.023.366.510.174.98

10.372.030.500.08

100.172.39

b0.10

c0.1396.7

193

29

3931

752.45.5

4.91.80.752.70.5

0.6

2.10.32

0.9<O.l

390.36

220.100.1481.23

762.225.830.8885.061.770.672.40.442.90.672.0

1.95

0.93

0.13

a0.236.2

16941.7

0.2819.20.100.131.23

2138

3504631PM9019.276.6

2.226.081.135.932.280.832.810.543.280.751.880.321.900.280.031.09

0.070.670.173.54

54.540.85

16.962.936.880.175.00

10.341.930.460.08

100.142.47

b<O.l

c0.1695.4

159

26

3930

701.95.3

4.51.70.742.60.5

0.6

2.00.31

0.9<O.l

410.37

210.0690.1351.05

782.246.51.015.832.060.762.460.483.170.712.1

2.1

0.98

0.11


primarily responsible here for the depletions of immobile HFSEs suchas Zr, Hf, Nb, Ta, Ti, and REEs, plus also LILEs. The phases of multi-stage melt extraction are specifically linked to the coupled backarc-arc dynamic systems.

2. Modification of this mantle wedge by replenishment (addition)of "mobile" LILEs (e.g., K, Rb, Cs, Sr, Ba, Pb, and U) derived fromdehydration reactions within the subducting slab. Moreover, theimprints of such mantle wedge "metasomatism" will be enhanced byprior depletion through melt extractions from this mantle wedge.

Modeling of HFSE and LILE Source Depletion

Depletions of HFSEs, especially Nb, Ta, and Ti relative to N-MORB, have long been recognized as characteristic of arc-type mag-mas (e.g., Chayes and Velde, 1965; Gill, 1981), although no consen-sus has been reached as to the cause(s) of these geochemical signa-tures. The previous discussion suggests that such features are source-inherited and develop as a consequence of multistage peridotitic

source depletion, which, in principle, should be amenable to numeri-cal modeling.

The results of two pairs of calculated fractional melting modelsare illustrated in Figure 21 (Models \-A), based on spinel-pyroxeneperidotite and amphibole peridotite model sources. Mineral phaseabundances are from McKenzie and O'Nions (1991), and the as-sumed nonmodal phase melting ratios are listed on the figures. Initialmelt fractions, melt increments, and amount of melt remaining inresidue are all assumed to be 0.01. Data are plotted as element ratiosfor each melt fraction, normalized to the starting composition. Ele-ment orders are the same as in previously presented spidergrams,except for Sr. The two sets of models differ in respect to the partitioncoefficient (KD) values used, especially for REEs (the choices ofwhich clearly determine the outcome of the models). Models 1 and 2utilize slightly modified KD values from basaltic andesites listed inEwart and Hawkesworth (1987), except Ti (taken from McKenzie andO'Nions [1991] for all four models). In Models 3 and 4, KD valuesare slightly modified from Ewart and Hawkesworth (1987) and McKen-

394


2 L2 L

5 -

Valu Fa

KTJ

ELSC

CLSC

5 0 6 0

\//Λ

V~X

p P.,

Site 837

Unit 1

Site 836Unit 5

Site 836Unit 4

Site 836Unit 3

π π r/

7/A

/ / / / A Y~TZ\ V^\ F]

Site 835

Unit 1

Site 834

Unit 13

Site 834Units 2 and 5

70 80

% An (mol.)90 100

Figure 2. Compilation of Plagioclase phenocryst and groundmass compositions, based on electron microprobedeterminations, in lavas from Sites 834 to 837. The published ranges of compositions of plagioclases fromCLSC, ELSC, KTJ, and VF are shown for comparison.

395


Table 5. Major element and new trace element analyses of modern Tonga-Kermadec lavas.

Island:Sample:


TiO2

A12O3

Fe2O3

FeOMnOMgOCaONa2OK2OP2O5TotalLOI



a0.042

14580.4760.080.151.2

2362

315

41677217.247.9

1.052.280.371.980.750.311.120.221.480.320.980.151.030.170.030.51

0.050.150.234.5

Tonga ArcTafahiT116

52.810.36

16.945.374.900.186.07

12.020.960.150.04

99.800.55

b c<O.l 0.106

2.3108

13.80.557.30.0570.1061.24

50

4540

55 45.51.1 0.932.3 2.53

0.385[1.1] 2.070.6 0.750.31 0.3211.1 1.05

[0.3]1.43

0.3 0.3120.946

1.1 0.950.17

<O.l 0.344<O.l

0.14

Tonga ArcFonualei

a0.18

1130537

1.3921

0.340.322.63

6190

2432

11318.2

186.13.37.641.276.292.090.722.670.493.050.651.950.32.090.350.071.18

0.180.440.197.4

F31

60.570.66

14.674.176.160.212.607.412.700.720.17

100.040.46

b0.35

4

3523

1803.48.1

6.31.90.692.30.5

0.6

2.10.33

0.7<O.l

Tonga Arc

a0.137

21529

0.61170.190.231.67

2443

320

391609016.8

1362.365.951.065.41.840.662.360.432.780.591.80.271.90.320.091.07

0.910.630.186.1

LateL3

54.580.60

15.864.026.850.204.94

10.261.690.560.09

99.650.13

b0.20

38

4837

1101.95.0

3.61.30.551.80.4

0.5

1.70.27

0.5<O.l

Tonga Arc

a0.79

1414046

0.4821

0.260.304.44

53313175

28885810.9

3623.137.231.195.641.880.712.290.422.640.591.700.281.870.290.041.38

0.141.210.22

11.1

Metis11108

64.250.39

12.531.365.270.125.157.032.600.910.07

99.680.91

b0.70

250

3227

3303.07.0

4.91.50.462.00.4

0.6

1.90.30

0.90.75

Tonga ArcKao104C

53.650.83

16.321.5

8.720.175.83

10.182.310.340.14

99.990.03

a b0.12 0.155.1

22351.7

0.418.40.160.232.71

2139 34

33847 4140 38778115.8

137.4 1203.1 2.68.48 7.21.548.15 6.32.73 2.10.95 0.833.47 2.80.62 0.63.880.8 0.82.380.362.45 2.20.4 0.340.041.65 0.9

<O.l0.10.40.174.5

Tonga ArcTofuaFN32

56.910.77

14.773.258.010.193.818.852.220.580.11

99.470.45

a b0.17 0.307.6

20742.6

0.3419.60.190.22.63

1015 11

39250 4136 35

123107

17.5173.4 170

2.27 2.45.93 6.21.055.61 5.22.09 1.90.75 0.742.97 2.70.57 0.53.660.77 0.82.340.362.42 2.30.39 0.360.031.25 0.8

0.100.050.440.214.7

Tonga ArcTofuaFN32

56.910.77

14.773.258.010.193.81

2.220.580.11

99.470.45

a b0.17 0.307.6

20742.6

0.3419.60.190.22.63

1015 11

39250 4136 35

123107

17.5173.4 170

2.27 2.45.93 6.21.055.61 5.22.09 1.90.75 0.742.97 2.70.57 0.53.660.77 0.82.340.362.42 2.30.39 0.360.031.25 0.8

0.100.050.440.214.7


zie and O'Nions (1991), the latter authors using very low values forLREE for olivines and pyroxenes. The KD values are listed in Table6. The progressive increments of fractional melts will, in principle,simulate the changing melt and source compositions during multi-stage magma extraction (initially taken as N-MORB).

Results indicate that both spinel-pyroxene peridotite models pre-dict preferential Nb-Th-Ti depletions at melt fractions exceeding 0.10(Models 1 and 3), together with even stronger LILE depletions (em-phasizing the necessity of subduction-related replenishment of theseelements in such models for the arc magmas; cf. Figs. 16-19). Themajor difference between Models 1 and 3 is in the extent of LREEdepletion, which is extreme in Model 3. Such extreme LREE deple1

tions are not observed in arc magmas, suggesting that, in the contextof the present models, either the very low KD values for the LREEsare inappropriate or the LREEs are also selectively replenished withLILEs from the subduction source. The amphibole peridotite frac-tional melting models (Models 2 and 4) do not predict the same degreeof Nb depletion, although they still produce limited Ti depletion, as a

result of the effective buffering of Nb, and to a lesser extent Ti, byamphibole. The LILEs exhibit strong preferential depletions, as inModels 1 and 3.

Thus, the models point to the plausibility that multistage magmaextraction from spinel-pyroxene peridotite (in which amphibole is nomore than a minor phase) has the capability of producing melts withprogressive selective depletion in Nb and Ti; other HFSEs will alsobe depleted but at a less rapid rate. The obvious implication, which isconsidered relevant to the Lau-Tonga-Kermadec arc-backarc system,is that the further the multistage melt extraction processes haveadvanced, the more extreme are the relative HFSE depletions.

Geochemical Evidence for Variable Source Depletion

Ewart and Hawkesworth (1987) utilized parameters such as TiO2/CaO and TiO2/Al2O3 as measures of source depletion, and the ratiosZr/Ba and Sr/Nd as convenient trace element ratios with which toillustrate the balance between "immobile" element depletion and su-

396



Island:Sample:

Tonga ArcHunga Ha'apai

HH1


TiO 2

A12O3

Fe2O3

FeOMnOMgOCaONa2OK2Op

2 o 5TotalLOI

54.560.49

18.562.086.740.164.17

11.331.780.240.06

100.170.22



a b0.13 0.154

185240.52

160.140.141.09

2654 42

26041

30 291307518.1

141 1101.8 1.44.59 3.10.824.34 2.61.56 1.10.58 0.462.16 1.70.4 0.32.660.58 0.51.740.271.88 1.50.31 0.260.030.91 0.3

0.150.060.440.234.9

TongaNiua fo'ou

N107

49.321.78

15.734.976.730.216.08

11.183.490.240.15

99.880.22

a b0.04 <O.l3.4

170120

4.51320.110.371.27

38215 20026550 5046 44738320.645.8 60

5.74 5.915.5 16.62.61

13.6 13.44.61 4.11.61 1.666.42 5.81.2 1.17.361.52 1.54.440.674.31 4.30.68 0.620.323.34 2.8

<O.l0.080.190.236.0

KermadecsRaoul7101

50.980.85

18.792.627.680.19

4.1912.282.060.280.05

99.971.09

a b0.11 0.304.5

200320.4

210.120.311.96

1536 24

36039

40 39878321.391.8 802.52 2.66.40 6.81.115.84 5.42.030 1.90.730 0.782.67 2.60.51 0.53.170.67 0.71.990.302.05 2.10.34 0.320.051.19 0.5

0.100.050.260.204.7

KermadecsRaoul23376

53.900.64

14.002.187.780.187.22

11.312.050.290.07

99.711.09

a b0.11 0.204.3

15344.90.46

16.10.140.311.72

196 17030147 4544 43

16.296.5 1002.71 2.77.01 7.51.165.84 5.51.94 1.80.66 0.692.53 2.40.47 0.62.960.62 0.61.840.291.95 1.90.32 0.300.041.27 0.8

0.050.260.105.2

KermadecsMacauley

10415

49.040.55

17.412.176.650.178.31

13.771.320.140.01

99.540.47

a b0.05 0.101.6

185150.22

130.050.131.58

49150 150280

4845 43846015.350.8 50

1.23 1.23.28 3.20.573.14 2.41.15 1.00.47 0.471.55 1.50.29 [0.4]1.820.37 0.41.090.161.06 1.20.17 0.190.020.57

0.100.030.200.224.7

KermadecsL'Esperance

14837

51.871.01

18.265.596.890.244.999.311.340.300.06

99.860.44

a b0.17 0.456.70

210450.41

200.190.512.98

1523 12

36042

42 40429921.8

141.40 1303.12 3.17.87 8.11.336.79 6.42.31 2.00.90 0.91

2.93.11 0.60.593.76 0.62.310.352.42 2.20.39 0.350.041.28 0.8

0.100.240.420.194.1

perimposed LILE enrichment. Plots of TiO2/CaO vs. Zr/Ba (Fig. 22)and Sr/Nd (Fig. 23) present data from all major subprovinces of theregion, again however, only including rock data for which MgO >4%. For reference, Figure 22 also shows the ranges of TiO2/CaO ofthe experimental equilibrium melts from model Hawaiian pyrolite(undepleted), but rather high TiO2) and Tinaquillo lherzolite (de-pleted) sources (Falloon et al., 1988), which clearly confirm the lowTiO2/CaO ratios of melts to be expected from relatively refractoryperidotitic sources.

Considering first the Leg 135 site data in Figure 22, there is a cleartrend of decreasing TiO2/CaO and Zr/Ba ratios going from Site 834,through Sites 835 and 836, to Site 839. This trend is matched by thedata fields for the modern Lau spreading centers in which there is aparallel trend of decreasing ratios from CLSC to ELSC and KTJ toValu Fa. These trends are here broadly correlated with both increas-ing source depletion, and increasing LILE source "metasomatism"toward Site 839 and Valu Fa. Reference to the fields for the modernarc volcanoes (shown, in part, in the inset to Fig. 22) and the boninitesindicate a clear continuation of these "depletion" trends through Ton-ga to the boninites. This plot, in fact, suggests a progressive evolution

of the Tongan lavas, with those of 'Ata and Kao defining the more"fertile" end of the data array. The Kermadec volcanoes exhibit morevariable trends, consistent with their greater variability of bulk chem-istry; the lavas of the southern Kermadec volcano of CEsperance areagain distinct from those of the northern Kermadec arc. Finally,attention is drawn to the Lau Ridge volcanics, namely the Lau andKorobasaga Volcanic Groups. Although the latter are more pro-nounced in their arc-like geochemistry, as noted by Cole et al. (1990),neither group exhibits the degree of depletion implied by the data forTonga, most of the Kermadec lavas, and the boninites.

Figure 23, the complementary Sr/Nd plot, reveals a similar patternof compositional changes. For example, the change toward "arc-like"chemistry is apparent in the Site 839 and Valu Fa data compared tothe Sites 834, 835, the CLSC, and ELSC. Similarly, the arc lavas andboninites exhibit overlapping compositional characteristics, the maindifference being that the boninites do not extend to the same highSr/Nd ratios as the northern Tongan lavas.

As a further illustration of the inferred systematically changingsource depletion characteristics, Figure 24 illustrates the changingratios of Zr/Sm. According to the data of Sun and McDonough (1989),

397


Niua fo'ou

Valu Fa

- ELSC

—< KTJt CLSC

αEcc

CO

Φ

E

% Fo (mol.)

Figure 3. Compilation of olivine phenocryst and microphenocryst composi-tions, based on electron microprobe determinations, in lavas from Sites 834 to836. The published ranges of compositions of olivines from CLSC, ELSC,KTJ, VF, and Niuafo'ou are also shown for comparison.

these two elements behave coherently in N-MORB, enriched mid-ocean-ridge basalts (E-MORB), and ocean-island basalt magmas(OIB; with 87Sr/86Sr of approximately 0.7035), the averaged Zr/Smratios lying close to 28. The Lau-Tonga-Kermadec data indicate agenerally decreasing Zr/Sm ratio that drops to between 10 and 20 withdecreasing Zr/Ba. It is important to note that fractional crystallizationhas only minor effects on the Zr/Sm ratios, as shown by the smallchange in the matrix Sample 135-839B-25R-1, 27-32 cm, of 17.4compared with the whole-rock value of 19.2.

The experimental data of Falloon et al. (1988) provide importantconstraints on the bulk compositions of liquids produced by equilib-rium partial melting of model peridotite sources that are undepleted(Hawaiian pyrolite) and depleted (Tinaquillo lherzolite). A majorconclusion is the confirmation of the relatively silica-saturated com-positions of the melts derived from the more refractory sources.Figures 9-11 compare the CaO, Na2O, and TiO2 abundances of theseexperimental liquids; the data clearly show the lower Na2O, TiO2, andhigher CaO contents of the Tinaquillo equilibrium melts. Thesetrends, particularly Na2O and TiO2, are entirely consistent with thedirections of "depletion" shown by the trace element data. The surveyof phenocryst mineralogical compositions, previously presented, arealso consistent with mineralogically heterogeneous sources, due tovarying prior melt extraction processes. For example, amongst theSite 839 lavas, the highly magnesian olivines (Unit 3) and calcicplagioclases (Unit 9) indicate relatively refractory sources, as do theCr-rich spinels in Units 1 and 3 (Allan, this volume).

In Figures 6 and 7, the normative compositions of the experimentalliquids are shown projected (from Plagioclase) into the silica-olivine-clinopyroxene, and from clinopyroxene into the olivine-plagioclase-silica systems. The data are approximately contoured according to theresidual phases in equilibrium with these liquids. These data suggestthat the less silica-saturated to silica-undersaturated compositions asfound, for example, in Sites 834, 836, and the CLSC and KTJ, aremore consistent with less depleted, MORB-like sources. The compo-sitional fields, however, for the northern Tonga boninites, and the Site839 data, imply refractory sources, possibly in equilibrium with anear-harzburgitic mineralogy (e.g., Duncan and Green, 1987). It isfurther possible that the small shifts in the compositional fields towardolivine, as seen for example in the CLSC, ELSC, ILSC, and Valu Fadata, may also reflect changing source characteristics toward slightlymore refractory chemistries.

In view of the previous interpretations of the existence of magmasource heterogeneities within the coupled Lau-Tonga mantle, it isrelevant to view these geographically. Thus, in Figure 25, the rangesof Zr/Ba ratios observed in the various volcanic zones and centers areplotted, including the Leg 135 drill sites (the ratios listed in Table 7).As previously discussed, Zr/Ba ratios are inferred to reflect melt sourcecharacteristics, are only slightly affected by moderate degrees of frac-tional crystallization, and are readily measured by XRF techniques(thus providing a good data set); these ratios may, however, be modifiedby magma mixing and assimilation-fractional crystallization processes(e.g., Hergt and Nilsson, this volume).

Figure 25 shows that within the Lau Basin, Zr/Ba ranges are greater,and generally higher than observed in the arc lavas of the Tonga andLau ridges, but that a southerly decrease of Zr/Ba occurs, most notablyat Site 839 and Valu Fa. On the Lau Ridge, the LVG possess ratiossignificantly higher than either the associated KVG or the modernTofua Arc lavas. Along the Tofua Arc, there is an overall trend ofnortherly decreasing Zr/Ba ratios (especially noting the lower ratiolimits), with the exception of the youthful volcano of Kao in the centralTofua Arc. Comparing the arc volcanos of the Lau and Tonga ridges,the LVG magma sources appear less depleted than those beneath theTofua Arc, although the younger KVG of the Lau Ridge exhibit similarZr/Ba ratios to those of Tonga. It has already been shown, however,that both arc-related volcanic groups from the Lau Ridge have muchless depleted HFSE abundances than occur in the Tofua Arc magmas(cf. Fig. 19). It is thus thought that magma sources beneath the Lau

398


Site 834Units 2 and 5coarse gm

(a)

Site 834Units 8 and 9BPh and gm

(C)

Mg — Fe+Mn

837Unit 1Ph and Mphto coarse gm

Site 836Unit 4Mph to coarse gm

— Fe+Mn

Figure 4. Compilation of pyroxene compositions, based on electron microprobe determinations, in lavas from Sites 834 to 837 (Figs. 4A-4I). Data plotted in termsof Ca, Mg, and Fe+Mn (atomic %). Also shown (Figs. 4J^L) are comparative published data from CLSC, ELSC, and VF. Phenocryst/microphenocryst (Ph) dataare plotted as solid circles, whereas groundmass (gm) phases are plotted as crosses.

Ridge were less refractory than currently exist beneath the modernTonga Ridge.

The overall pattern emerging is that within the Lau Basin, theELSC-Valu Fa propagator is becoming strongly "arc-like" geochemi-cally as it approaches the modern Tofua Arc, with the Leg 135 sitesalso becoming more "arc-like" southeastward across the older, west-ern segment of Lau Basin crust, presumably also reflecting increasingproximity toward the arc environment before the renewed opening ofthe central eastern Lau Basin by the ELSC propagator system. Acompilation of the ranges of significant element ratios from thevarious subregions of Lau-Tonga illustrating the regional and tempo-ral changes of geochemistry, is presented in Table 7.

A POSSIBLE MODEL

Parson and Hawkins (this volume) have developed a two-stagemodel of Lau Basin evolution, the following summarized from theirpaper: The active Lau-Fiji-Tonga proto-arc underwent protracted

extension commencing sometime after 10 Ma, resulting in openingof a largely amagmatic basin floor characterized by irregular horst-and-graben topography, through the period before 5.5 Ma. At approxi-mately 5.5 Ma, true backarc spreading initiated in the north centralLau Basin and propagated southward from the Peggy Ridge, formingthe ELSC, the southern limit of which is now the Valu Fa Ridge. Axialspreading results in fan-shaped opening of the eastern portion of theLau Basin. A later southward moving propagator, the CLSC, com-menced from the southeastern limit of the Peggy Ridge between 1.2and 1.5 Ma. Isotopic data by Hergt and Hawkesworth (this volume)has shown that an influx of asthenosphere with Indian Ocean mantleisotopic affinities accompanied the younger phase of true, backarcspreading, displacing older asthenosphere with Pacific Ocean mantleisotopic affinities beneath the backarc region. As previously noted,the isotopic data further indicate that mixing between the two as-thenospheric end-members has occurred. The implications, therefore,point to dominantly "Pacific" mantle beneath the Lau Ridge, thewestern part of the Lau Basin, and the Tonga (Kermadec) Ridge, with

399


80

70

üO

CCΦO

+ 60U-

D)

ö)

50

40

839B-25R-1; 33-41 (Unit 3)

839B-25R-1; 38-41 (Unit

839B-23R-1; 0-6 (Unit

839B-20R-1; 29-35 (Unit 3)

836A-9X-1; 138-143 (Unit 4)836B-6R-1; 105-110 (Unit 4B)

(Unit 3) 836A-4H-CQ0-13 ^ ^(Unit 7) 834B-34R-1; 76-80 —^~(Unit 7) 834B-33R-2; 105-110(Unit 1) 839B-13R-2; 25-30 '

(Unit 13) 834B-57R-1; 126-128(Unit 8) 834B-35R-1; 58-60(Unit 1) 835B-4R-1; 134-141

3)

834B-25R-1; 106-110 (Unit 7)* 839B-25R-1; 27-32 (Matrix)

835B -7R-1; 15-22 (Unit 1)

834B-31R-3; 73-78 (Unit 7)

* 834B-37R-1; 36-42 (Unit 9B)

834B-15R-2; 47-56 (Unit 6)

836B-9M-1; 117-122 (Unit 5)

834B-55R-1; 23-25 (Unit 12)

60 70 80 90 100

Mg / Mg + Fe - OlivineFigure 5. Ranges of olivine compositions (microprobe determinations; expressed as atomic% Mg/(Mg + Fet) in lava samples from Sites 834, 835, 836, and 839,compared to their bulk-rock (and one-matrix) compositions, expressed as Mg values (atomic% Mg/(Mg + Fe2+), assuming Fe2O3/FeO = 0.2). Three sets ofFe-Mg partition coefficient curves are shown.

"Indian" mantle beneath the wedge-shaped eastern portion of the LauBasin, at least south of the Peggy Ridge. The systematics north of thePeggy Ridge are still unclear.

The above interpretation also clearly implies that the western"horst-and-graben" segment of the Lau Basin, before propagatoractivity, lay adjacent to the Tonga Ridge and arc, and thus significantlycloser to the subduction zone. This is believed to be the reason whythe lavas from Sites 835 and 837-839 exhibit such strong, althoughvariable, arc-like geochemical signatures. In fact, it is proposed thatSite 839 may actually represent an abandoned arc volcanic constructbelonging to an early phase of the Tongan arc volcanism, now stranded

within the Lau Basin by the subsequent opening up the eastern part ofthe Lau Basin by propagator activity (see Ewart, Hergt and Hawkins,this volume, for further discussion of Site 839).

Arc-related volcanism on the Lau Ridge occurred continuouslyfrom 14 to 5.4 Ma (Lau Volcanic Group), and 4.4 to 2.4 Ma (Koro-basaga Volcanic Group), clearly overlapping the earlier phase of LauBasin extension (Cole et al., 1985,1990). However, data compiled byJarrard (1986) indicate that the age at which the tip of the Tongan slabbegan subduction was approximately 9.6-10 Ma, suggesting that thepresent Tongan (and Kermadec) subduction zone was not associatedwith at least the earliest stages of Lau Ridge arc volcanism.

400


vqtz/OtZ

( a )Sites

Δ - 834O - 835+ - 836× - 837α - 838

King's Triple

Oliv Oliv Oliv

Qtz

( f )

Oliv

Experimental equilibriumpartial melts

D

X

•

®

a

- 5kb- 10kb- 15kb-20kb-30kb

oliv+opx+cpx+ploliv+opx

\ ^ ' f l S - oliv+pl

θ \ oliv | oliv+opx+cpx

\ oliv\OPX|

/ ×ii

Hawaiian pyroliteequilibium melting

J Cpx

( g )

Oliv

Qtz

Experimental equilibrium partial melts

Δα×•θ

a

- 2kb- 5kb-10kb-15kb-20kb-30kb

Tiπaquillo Iherzoliteequilibrium melting

Cpx

Figure 6. Compositions of lava from Sites 834 to 839, the Lau Basin spreading centers, the modern Tongan and Kermadec axial volcanoes, and the northern Tongan

boninites, plotted in terms of olivine (Oliv), silica (Qtz), and clinopyroxene (Cpx) (projected from Plagioclase). The solid lines are the 1-atm. cotectics and reaction

curves (after Grove and Bryan, 1983), as are the projected 10- and 20-Kbar cotectics. The dashed curves are inferred cotectics and reaction curves, at P H 0 = 1000

bars, after Grove et al. (1982). Projection scheme calculation after Grove et al. (1982). Data sources for this, and following figures, listed in text. In Figures 6F

and 6G, the experimental equilibrium liquid compositions, at various pressures, are plotted after Falloon et al. (1988); the residual mineral assemblages in

equilibrium with these liquids are approximately contoured.

401


Plag

834

835 to838

Plag

Modern Tonga -Kermadec axial

volcanoes

Oliv Qtz Oliv Qtz

Plag Plag

D

X•

θ

a

- 2 K b- 5 Kb- 1OKb- 15Kb- 20 Kb- 30 Kb

o l i v + 0 '

Tinaquillo Iherzoliteequilibrium melting

Oliv Qtz Oliv Qtz

Figure 7. Compositions of lavas from Sites 834 to 839, the Lau Basin spreading centers, the modern Tonga-Kermadec axial volcanoes, and the northern Tonganboninites plotted in terms of olivine (Oliv), silica (Qtz), and Plagioclase (Plag), projected from clinopyroxene. Projection scheme after Grove et al. (1982). InFigures 7C and 7D, the experimental liquid compositions, at various pressures, are plotted after Falloon et al. (1988), and the residual mineral assemblages thatare in equilibrium with these liquids are approximately contoured.

The primary event, therefore, which is thought to have causedinitiation of Lau-Tonga extension, and subsequent backarc spreading,is an abrupt jump in the position of the subduction zone. The reasonfor such a jump would plausibly be extended slab rollback. If correct,such a jump is inferred to have caused a change in mantle streamlineflow resulting in a tensional regime sufficient to initiate rifting of theformerly continuous Lau-Tonga ridge system (see Fig. 26).

Analyses of deep seismic data for the Tonga-Kermadec region byGiardini and Woodhouse (1984) show a zone of very contortedseismicity extending to 650 km, which these authors correlate with astrongly sheared and displaced subducted slab. These seismic profilessuggest a possible break in the zone at near 400 km depth; if thisrepresents the front of a newer descending slab, then it is consistentwith a subduction jump at ~6 Ma (using the data from Jarrard, 1986),

and would imply that the deeper seismicity would represent theremnants of the previously subducted slab. It is significant that theseismic profiles of Louat and Dupont (1982) also show slab imbrica-tion at depths of >500 km. Alternatively, the inferred age of 9.6-10Ma for the whole of the Tongan slab is still consistent with the inferredage of initial Lau Basin extension (between 5.5 and a maximum of10 Ma; Parson and Hawkins, this volume), implying that the original,but now inactive, Lau-Tonga Ridge subducted slab is no longer detect-able through active seismicity.

Figure 26 summarizes the inferred evolution of the Lau Basin. Thesequence of mantle streamlines is based on the premise that the proc-esses of backarc magma generation are responsible for the inferreddepletion of the mantle wedge, which, in turn, convects into the regionbeneath the active arc volcanism from which arc magmas (and bonin-


10Modern

Kβrmadβc

Volcanoes

To ?5

1 0 -

δ -

ModernTonganAxialVolcanoes

10

/

Korobasaga

r H i Volcanic Group

m (Lau Islands)

10 15

Stations 2 1 , 22, 23NorthTonga Ridge

LauVolcanicGroup(Lau Islands)

15

Site 839

(including glasses)

15

Sites 837 and 838

10 15

Site 836

10

Site 835

10 1515

10

J\Site 834

5 10

% MgO15 5 10

% MgO15

Figure 8. Histograms illustrating the distribution of %MgO in lavas from Sites 834 to 839, the Lau Basin Spreading Centers and the Lau Ridge, the

modern Tongan and Kermadec arc volcanoes, and the northern Tongan boninitic series.

403


15

%

CaO10

Valu Fa, 15

1 0 -

Expβrimβntalequilibriumpartial melts

TinaquilloIherzolite

Δ

π×•+B

2 Kb5 Kb

10 Kb15 Kb20 Kb30 Kb

(d)

Hawaiianpyrolite

15

15

CaO10

(b),

•

o

Valu FaELSCandILSCCLSC

19

15

%

CaO10

5

-

-

(a)

^ j f ^ ^ Δ W " U ^

1

Δo+X

π•

i

Sites 834 to 839

"~-——-s3T834835836837838839

—

15

10

Modern Tonganvolcanoes

Site839

Ce)

• Modern Tongan axial volcanoes

× Site 841 dykes/sills

Kermadecs

Site

(f)

10

% MgO15 20 10 15 20

% MgO25 30

Figure 9. Compilation of CaO-MgO data comparing lavas of the modern Lau Basin spreading centers, Sites 834—839 and 841, the modern Tonga-Kermadecvolcanoes, and the northern Tongan boninites. Also shown are the experimental equilibrium liquids, at various pressures, after Falloon et al. (1988). The tie-linejoins a coexisting whole rock-matrix pair.

Table 6. Partition coefficients used in fractional melting models (Fig. 21, Models 1-4).

Rb, KBaSrThNbLaCeNdZrHfSmEuTiYbLu

Olivinea

0.00020.00860.050.0140.010.00180.00180.00180.100.100.00190.00190.0060.00940.0118

Olivineb

0.00020.00010.00020.010.010.00050.00080.00130.0230.0220.00190.00190.0060.0040.0048

Orthopyroxene3

0.00060.0140.0070.00010.020.2380.200.1440.050.010.1440.1410.0240.2570.28

Orthopyroxene

0.010.010.010.050.030.0290.0310.0340.040.040.0470.060.0240.2440.300

Clinopyroxene11

0.0110.0770.0670.1520.050.1580.2230.360.250.2330.360.4240.100.5630.55

Clinopyroxeneb

0.0040.0050.0860.050.040.0820.0960.1820.140.210.2610.260.100.2270.19

Spinel0

0.00010.00010.00010.020.010.0290.0320.0410.020.020.0410.0530.0480.100.105

Spineld

0.00010.00010.00010.020.010.0290.0320.0370.020.020.0530.0630.0480.110.091

Amphibole6

0.20.330.120.600.800.170.260.440.400.500.760.880.690.590.51

Notes: Partition coefficents slightly modified from McKenzie and O'Nions (1991) and Ewart and Hawkesworth (1987).a Models 1 and 2.b Models 3 and 4.c Model 1.d Model 3.e Models 2 and 4.

404


6.0

5.0

4.0

%

Na2O3.0

2.0

1.0

5.0

4.0

Na2O3.0

2.0

1.0

4.0

Na2O3.0

2.0

1.0

King's TripleJunction

5.0

4.0

r Experimentalequilibriumpartial melts

2.0

1.0

Valu Fa

5.0

D

Sites 834

\ Δ

\ °\ +\ ×

\ α

\

to 839

834835836837838839

4.0

3.0-

2.0-

1.0-

+

•t

Δ

α×•+

2510152030

KbKbKbKbKbKb

LU [JHawaiian pyrolite

*(d)

TinaquilloIherzolite

• Tongan boninitic suite

ModernTonganvolcanoes

Modern Tongan axial volcanoes

+ Site 841 dykes/sills

Site 839

5 10 15

% MgO20 0 10 15

% MgO20 25

Figure 10. Compilation of Na2O-MgO data, as in Figure 9.

405


2.0

+ 836× 837D 838• 839

Experimentalequilibriumpartial melts

Hawaiian pyrolitβ

Δ

D

×•+B

2

5

10

15

20

Kb

Kb

Kb

Kb

Kb

30 Kb

×××× +B

Tinaquillo IherzoliteΔ

B++

r Modern Xpngan volcanoes

20 0

• Modern Tongan axial volcanoes+ Site 841 dykes/sills

ite 839

10% MgO

20

Figure 11. Compilation of TiO2-MgO data, as in Figure 9.

ites) are themselves derived. Thus, the most intensive zones of sourcedepletion (i.e., most refractory sources) are correlated with the widestzones of backarc magmatism, that is, toward the northern end of theTonga Ridge and increasing northward within the Kermadec Arc. It isfurther noted that there appears to be, and in fact is predicted to be, anet increase in the refractory character of the source of the arc magmastoward the east, with the Lau Ridge arc magmas sourced from lessrefractory mantle than those of Tonga. It is noted, however, that thestreamlines shown in Figure 26 do not coincide with those predictedby recent geophysical modeling (e.g., Spiegelman and McKenzie,1987), although both models still require focusing of streamlines intothe volcanic front. Such geophysically modeled streamline configura-tions are not easily reconciled into the coupled arc-backarc systemsinferred here to operate in the Lau-Tonga system.

The effect of increasing proximity to the modern Tonga subduc-tion zone, and the arc volcanism, is suggested as the reason for thechange of geochemical character of the ELSC lavas (weak arc signa-tures) toward the Valu Fa Ridge (strong arc signatures); the youngerCLSC magmas now occurring in the central Lau Basin are geochemi-cally nearer N-MORB. The development of arc-like signatures in thelavas from Sites 835, 837, and 838 are similarly attributed to theircloser proximity to the Tongan subduction zone during eruption, be-

fore the propagator activity displacing the arc and subduction zonefurther eastward.

It is further suggested that the development of the Lau Basin, withits consequent net eastward movement of the Tongan-Kermadec arcs,is just the latest stage in the progressive eastward fragmentation ofthe southwest Pacific arc systems, resulting in stranded extinct arcs(e.g., Loyality and Three Kings arcs) and accompanying basins (e.g.,South Fiji Basin), as outlined by Kroenke (1984).

ACKNOWLEDGMENTS

We thank the staff and crew of the JOIDES Resolution and OceanDrilling Program for their enthusiastic and skilled work during the Leg135 drilling. Analytical work on which this paper is based were con-ducted at the Departments of Earth Sciences at Monash University,University of Queensland, and Australian National University. Thanksare owed to the staff in these departments for extensive assistance. MrsE. Burdin, University of Queensland, drafted all figures. The manu-script received much technical and editorial assistance and improve-ment from reviews by Profs. J. Cole and J. Hawkins, Dr. C. J. Stephens,ODP editorial staff, and an unnamed reviewer. Part of this researchwas funded by a Special Project Grant, University of Queensland.

406


REFERENCES

Baker, RE., Harris, RG., and Reay, A., 1971. The geology of Tofua Island,Tonga. In Cook Bicentenary Expedition in the South-West Pacific. Bull.—R. Soc. N.Z., 8:67-69.

Bauer, G.R., 1970. The geology of Tofua Island, Tonga. Pac. Sci., 24:333-350.Boespflug, X., Dosso, L., Bougault, H., and Joron, J.L., 1990. Trace element

and isotopic (Sr and Nd) geochemistry of volcanic rocks from the LauBasin. In von Stackelberg, U., and von Rad, U. (Eds.), Geological Evolu-tion and Hydrothermal Activity in the Lau and North Fiji Basins, South-west Pacific Ocean (Results of SONNE Cruise SO-35). Geol. Jahrb.,92:503-516.

Chappell, B.W., and Hergt, S.M., 1989. The use of known Fe content as a fluxmonitor in neutron activation analysis. Chern. Geol., 78:151-158.

Chayes, R, and Velde, D., 1965. On distinguishing basaltic lavas of circum-oceanic and oceanic-island type by means of discriminant functions. Am.J. Sci., 263:206-222.

Cole, J.W., Gill, J.B., and Woodhall, D., 1985. Petrologic history of the LauRidge, Fiji. In Scholl, D.W., and Vallier, T.L. (Eds.), Geology and OffshoreResources of Pacific Island Arcs—Tonga Region. Circum-Pac. Counc.Energy Miner. Resour., Earth Sci. Sen, 2:379^14.

Cole, J.W., Graham, I.J., and Gibson, I.L., 1990. Magmatic evolution of LateCenozoic volcanic rocks of the Lau Ridge, Fiji. Contrib. Mineral. Petrol,104:540-554.

Collier, J., and Sinha, M., 1990. Seismic images of a magma chamber beneaththe Lau Basin back-arc spreading center. Nature, 346:646-648.

Duncan, R.A., and Green, D.H., 1987. The genesis of refractory melts in theformation of oceanic crust. Contrib. Mineral. Petrol, 96:326-342.

Ernewein, M., Pearce, J.A., Bloomer, S.H., Parson, L.M., Murton, B.J., andJohnson, L.E., in press. Geochemistry of Lau Basin volcanic rocks: influ-ence of ridge segmentation and arc proximity. In Smellie,. J. (Ed.), Volcan-ism Associated with Extension at Consuming Plate Margins. Geol. Soc.Spec. Publ. London.

Ewart, A., 1976. A petrological study of the younger Tongan andesites anddacites and olivine tholeiites of Niuafo'ou Island, S.W. Pacific. Contrib.Mineral. Petrol, 58:1-21.

Ewart, A., Brothers, R.N., and Mateen, A., 1977. An outline of the geology andgeochemistry, and the possible petrogenetic evolution of the volcanic rocksof the Tonga-Kermadec-New Zealand island arc. J. Volcanol. Geotherm.Res., 2:205-250.

Ewart, A., Bryan, W.B., and Gill, J., 1973. Mineralogy and geochemistry ofthe younger volcanic islands of Tonga, southwest Pacific. J. Petrol,14:429^165.

Ewart, A., and Hawkesworth, C.J., 1987. The Pleistocene-Recent Tonga-Ker-madec arc lavas: interpretation of new isotope and rare earth data in termsof a depleted mantle source model. J. Petrol, 28:495-530.

Falloon, T.J., and Crawford, A.J., 1991. The petrogenesis of high-calciumboninite lavas dredged from the northern Tonga ridge. Earth Planet. Sci.Lett, 102:375-394.

Falloon, T.J., Green, D.H., and Crawford, A.J., 1987. Dredged igneous rocksfrom the northern termination of the Tofua magmatic arc, Tonga andadjacent Lau Basin. Aust. J. Earth Sci., 34:487-506.

Falloon, T.J., Green, D.H., Hatton, C.J., and Harris, K.L., 1988. Anhydrouspartial melting of a fertile and depleted peridotite from 2 to 30 Kb andapplication to basalt petrogenesis. J. Petrol, 29:1257-1282.

Falloon, T.J., Green, D.H., McCulloch, M.T., 1989. Petrogenesis of high-Mgand associated lavas from the north Tonga trench. In Crawford, A.J. (Ed.),Boninites and Related Rocks: London (Unwin Hyman), 357-395.

Falloon, T.J., Malahoff, A., Zonenshain, L.P., and Bogdanov, Y, 1992. Petrol-ogy and geochemistry of back-arc basin basalts from Lau Basin spreadingridges at 15, 18, and 19°S. Mineral Petrol, 47:1-35.

Frenzel, G., Mühe, R., and Stoffers, P., 1990. Petrology of volcanic rocks fromthe Lau Basin, Southwest Pacific. Geol. Jahrb., Riehe D, 92:395^79.

Giardini, D., and Woodhouse, J., 1984. Deep seismicity and modes of defor-mation in Tonga subduction zone. Nature, 307:505-509.

Gill, J.B., 1976. Composition and age of Lau basin and ridge volcanic rocks:implications for evolution of an interarc basin and remnant arc. Geol. Soc.Am. Bull, 87:1384-1395.

Abbreviations for names of organizations and publication titles in ODP reference listsfollow the style given in Chemical Abstracts Service Source Index (published byAmerican Chemical Society).

, 1981. Orogenic Andesites and Plate Tectonics: New York (Sprin-ger-Verlag).

Grove, T.L., and Bryan, W.B., 1983. Fractionation of pyroxene-phyric MORB atlow pressure: an experimental study. Contrib. Mineral. Petrol, 84:293-309.

Grove, T.L., Gerlach, D.C., and Sando, T.W., 1982. Origin of calc-alkalineseries lavas at Medicine Lake Volcano by fractionation, assimilation andmixing. Contrib. Mineral. Petrol, 80:160-182.

Hart, S.R., and Reid, M.R., 1991. Rb/Cs fractionation: a link between granulitemetamorphism and the S-process. Geochim. Cosmochim. Acta, 55:2379-2838.

Hawke, M.M., 1983. Mineralogy of some igneous rocks from the KermadecIslands and Taupo Volcanic Zone, New Zealand [unpubl. Ph.D. thesis].Univ. of Auckland, New Zealand.

Hawkins, J.W., 1976. Petrology and geochemistry of basaltic rocks of the LauBasin. Earth Planet. Sci. Lett, 28:283-297.

Hawkins, J.W., and Melchior, J.T., 1985. Petrology of Mariana Trough andLau Basin basalts. J. Geophys. Res., 90:11431-11468.

Hofmann, A.W., 1988. Chemical differentiation of the Earth: the relationshipbetween mantle, continental crust, and oceanic crust. Earth Planet Sci.Lett, 90:297-314.

Jarrard, R.D., 1986. Relations among subduction parameters. Rev. Geophys.,24:217-284.

Jenner, G.A., Cawood, RA., Rautenschlein, M., and White, W.M., 1987.Composition of back-arc basin volcanics, Valu Fa Ridge, Lau Basin:evidence for a slab-derived component in their mantle source. J. Volcanol.Geotherm. Res., 32:209-222.

Jochum, K.P., Seufert, H.M., Spettel, B., and Palme, H., 1986. The solar-sys-tem abundances of Na, Ta, and Y, and the relative abundances of refractorylithophile elements in differentiated planetary bodies. Geochim. Cosmo-chim. Acta, 50:1173-1183.

Jochum, K.P., Seufert, H.M., and Thirlwall, M.F., 1990. High-sensitivity Nbanalysis by spark-source mass spectrometry (SSMS) and calibration ofXRFNbandZr. Chern. Geol, 81:1-16.

Johnstone, R.D., 1978. 'Ata, the most southerly volcanic island in Tonga. InLau-Tonga 1977. Bull—R. Soc. N.Z., 17:153-164.

Kay, R.W., and Hubbard, NJ., 1978. Trace elements in ocean ridge basalts.Earth Planet. Sci. Lett, 38:95-116.

Kroenke, L.W., 1984. Cenozoic Tectonic Development of the Southwest Pacific.U.N. Econ. Soc. Comm. Asia Pac, Suva, Fiji. CCOP/SOPAC, Tech. Bull, 6.

Loock, G., McDonough, W.F., Goldstein, S.L., and Hofmann, A.W., 1990.Isotopic compositions of volcanic glasses from the Lau Basin. Mar. Min.,9:235-245.

Louat, R., and Dupont, J., 1982. Sismicité de L'arc des Tonga-Kermadec. InEquipe de Géologie-Geophysique du Centre ORSTOM de Noumea. Con-trib. à 1'etude geodynamique du Sud-Ouest Pacifique, Trav. Doc. OR-STOM, 147:299-317.

McDonough, W.F., Sun, S.-S., Ringwood, A.E., Jagoutz, E., and Hofmann,A.W., 1992. Potassium, rubidium, and cesium in the Earth and Moon andthe evolution of the mantle of the Earth. Geochim. Cosmochim. Acta,56:1001-1012.

McKenzie, D., and O'Nions, R.K., 1991. Partial melt distributions frominversion of rare earth element concentrations. J. Petrol, 32:1021-1091.

Melson, WG., Jarosewich, E., and Lundquist, CA., 1970. Volcanic eruptionat Metis Shoal, Tonga, 1967-1968. Description and petrology. Smithson.Contrib. Earth Sci., 4.

Norrish, K., and Hutton, J.T., 1969. An accurate X-ray spectrographic methodfor the analysis of a wide range of geological samples. Geochim. Cosmo-chim. Acta, 33:431-453.

Oversby, VM., and Ewart, A., 1972. Lead isotopic compositions of Tonga-Kermadec volcanics and their petrogenetic significance. Contrib. Miner.Petrol, 37:181-210.

Parson, L., Hawkins, J., Allan, J., et al., 1992. Proc. ODP, Init. Repts., 135:College Station, TX (Ocean Drilling Program).

Parson, L.M., Pearce, J.A., Murton, B.J., Hodkinson, R.A., Bloomer, S.,Ernewein, M., Huggett, Q.J., Miller, S., Johnson, L., Rodda, P., and Helu,S., 1990. Role of ridge jumps and ridge propagation in the tectonic evolutionof the Lau back-arc basin, southwest Pacific. Geology, 18:470-473.

Reay, A., Rooke, J.M., Wallace, R.C., and Whelan, P., 1974. Lavas fromNiuafo'ou Island, Tonga, resemble ocean-floor basalts. Geology, 2:605-606.

Scholl, D.W., Vallier, T.L., and Maung, TU., 1985. Introduction. In Scholl,D.W., and Vallier, T.L. (Eds.), Geology and Offshore Resources of thePacific Island Arcs—Tonga Region. Circum-Pac. Counc. Energy Miner.Resour., Earth Sci. Ser., 2:3-15.

407


Spiegelman, M, and McKenzie, D.P., 1987. Simple 2-D models for meltextraction at mid-ocean ridges and island arcs. Earth Planet. Sci. Lett.,83:137-152.

Sun, S.-S., and McDonough, W.F., 1989. Chemical and isotopic systematicsof oceanic basalts: implications for mantle composition and processes. InSaunders, A.D., and Norry, MJ. (Eds.), Magmatism in the Ocean Basins.Geol. Soc. Spec. Publ. London, 42:313-345.

Sunkel, G., 1990. Origin of petrological and geochemical variations of LauBasin lavas (SW Pacific). Mar. Min., 9:205-234.

Tao, G.Y., Pella, RA., and Rousseau, R.M., 1985. NBSGSC—a fortran pro-gram for quantitative X-ray fluorescence analysis. NBS Tech. Note (U.S.),1213:124.

Taylor, S.R., and Gorton, M.P., 1977. Geochemical application of spark sourcemass spectrometry. III. Element sensitivity, precision and accuracy. Geo-chim. Cosmochim. Acta, 41:1375-1380.

Valuer, T.L., Jenner, G.A., Frey, R, Gill, J., Davis, A.S., Hawkins, J.W., Morris,J.D., Cawood, P.A., Morton, J., Scholl, D., Rautenschlein, M., White,W.M., Williams, R.W., Volpe, A.M., Stevenson, A.J., and White, L.D.,1991. Subalkaline andesite from Valu Fa Ridge, a back arc spreading centerin southern Lau Basin: petrogenesis, comparative chemistry, and tectonicimplication. Chern. Geol., 91:227-256.

Valuer, T.L., Stevenson, A.J., and Scholl, D.W., 1985. Petrology of igneousrocks from 'Ata Island, Kingdom of Tonga. In Scholl, D.W., and Valuer,T.L. (Eds.), Geology and Offshore Resources of Pacific Island Arcs—Tonga Region. Circum-Pac. Counc. Energy Miner. Resour., Earth Sci. Sen,2:301-316.

Viereck, L.G., Flower, M.F.J., Hertogen, J., Schmincke, H.-U., and Jenner,G.A., 1989. The genesis and significance of N-MORB sub-types. Contrib.Mineral. Petrol, 102:112-126.

Volpe, A.M., Macdougall, J.D., and Hawkins, J.W., 1988. Lau basin (LBB):trace element and Sr-Nd isotopic evidence for heterogeneity in backarcbasin mantle. Earth Planet. Sci. Lett, 90:174-186.

von Stackelberg, U., and Shipboard Scientific Party, 1985. Hydrothermalsulphide deposits in back-arc spreading centres in the southwest Pacific.Bundesanst. Geowiss. Rohstoffe Circ, 2:3-14.

, 1988. Active hydrothermalism in the Lau back-arc basin (SWPacific): first results from the Sonne 48 cruise (1987). Mar. Min., 7:431—442.

Date of initial receipt: 29 June 1992Date of acceptance: 14 June 1993Ms 135SR-141

APPENDIX

Analytical Techniques

Atomic Absorption Graphite Furnace Methods: Pb: Sample digested inHF/HNO3 and fumed to dryness 4 times with HNO3 to convert to nitrates.After making up to volume, analyzed in a Perkin Elmer HGA300 graphitefurnace and AS40 autosampler fitted to a Perkin Elmer 2380 AAS using peakarea measurement. A Pb hollow cathode lamp and deuterium backgroundcorrector were used, with matrix modification using NH4H2PO4, at a wave-length of 217.0 nM.

Be: 100 mg of rock powder digested in HF/HNO3 and fumed down 4 timeswith HNO3 to convert all fluorides to nitrates. Analyzed by a Perkin ElmerHGA300 graphite furnace with AS40 autosampler on a 2380 AAS. Theparameters used were as follows: 15-mA lamp current plus D2 backgroundcorrector at 234.9 nM; L'vov platform; 20 µL of sample plus 10 µL of 2500ppm Mg(NO3)2 used as matrix modifier. Detection limit approximately 0.05ppm Be in rock.

ICP-MS method: Approximately 0.1 g of sample was weighted out accu-rately into a 100-cm3 PTFE beaker. Samples were digested on a hot plate usingultrapure HF followed by HN03. The resultant nitrates were taken up in25-cm3 0.5% nitric acid. The samples were made up to 100-µg dm"3 withrespect to indium (internal standard) and analyzed by ICP-MS (PQ2+ Plas-maQuad VG Elemental, England) in "peak jumping mode." Standards of 0.05,0.1, and 0.2 mg BHVO-1 and RGM-1 were used to construct calibrationcurves. Experimental conditions were as follows: number of peak jump sweeps= 100, points per peak = 5, and Dae steps between points = 5.

Electron microprobe: Mineral analyses were performed on a newly in-stalled, fully automatic, JEOL Superprobe, at the University of QueenslandCentre for Microscopy and Microanalysis. Standards comprised international

oxide and mineral standards, 20- to 30-s count times, and 1-µm focused beam(10 µm for feldspars and glasses). Full ZAF correction procedures were applied.

X-ray Fluorescence: Major elements were analyzed on glass discs pre-pared from samples weighed with lithium borate flux. Trace elements wereanalyzed on pressed discs, prepared from 6 g of powdered sample. Methodsfollow those described by Norrish and Hutton (1969), with major element dataprocessing using NBS/TN-1213 software package (Tao et al., 1985). Absorp-tion coefficients are applied to the trace element analyses by calculation fromthe major element analyses with correction procedures following the Univer-sity of Cape Town software (A.R. Duncan, pers. comm., 1991).

INAA: Methods and instrumentation followed those described in Chappelland Hergt (1989).

SSMS Method: The method used here follows that of Taylor and Gorton(1977). About 150 mg of powder was mixed with the same amount of graphite,which contains Lu (used as an internal standard) and spikes for Ba, Nd, andPb. This mixture was then pressed into electrodes that were analyzed on theMS7 double-focusing mass spectrometer, which employs photoplates fordetection purposes. At least three photoplates were analyzed for each sampleand the results were pooled. Accuracy, based on replicate analyses of interna-tional standards, is ±10% for all elements except Rb and Sr, which are accurateto within 20%. Precision is generally better than ±5% (2 sigma), althoughelements at very low or very high concentrations will have poorer precisionbecause of the fewer number of data points that are pooled—ideally, aminimum of 20 separate determinations are used in the final results. Recentimprovements to the MS7 have resulted in resolution of an interference on93Nb (because of 29Si16O4; Jochum et al., 1990), which, if left unresolved,would most greatly affect samples with low Nb concentrations (i.e., <l ppm).

Comparison of Analytical Methods

Six samples were measured by three separate methods (i.e., ICP-MS,INAA, and SSMS), allowing comparison of results for overlapping elements,namely, Ba, Hf, and the REEs.

For the REEs, all three analytical techniques yield results that are generallywithin 10% of one another, with no systematic bias displayed by any individualtechnique. An exception to this is Ce, where the SSMS data appear to besystematically high by 5%-15% when compared with those from ICP-MSor INAA.

The Ba data are within 10% for SSMS and ICP-MS. The INAA Ba issimilar to that from ICP-MS and SSMS at higher concentrations; however,below -50 ppm the INAA data tend to be higher than either ICP-MS or SSMSdata. This general trend is preserved when one examines the other samples forwhich both ICP-MS and INAA data are available; the INAA Ba is generallyhigher (by up to a factor of 3) than the ICP-MS Ba for rocks with <50 ppm.

Hf shows the poorest comparison between the three techniques, withoverall variation of up to a factor of 2, but with most results falling within±20%. The ICP-MS data are consistently higher than the SSMS data. TheINAA data are similar to the SSMS data at high Hf concentrations (>0.9 ppm),but they show much lower values at lower levels compared with either theICP-MS or SSMS data. Some estimate of the relative accuracy of thesedisparate measurements may be gained by comparing Zr/Hf ratios. For thesesamples, Zr was measured by XRF and SSMS, and the data generally repro-duced within 10%. Thus, little variation in the Zr/Hf ratio will be producedbecause of variation in the Zr data. The Zr/Hf ratios measured by the ICP-MSare between 36 and 39, whereas the SSMS Zr/Hf ratios lie between 35 and 44,with the exception of one sample (135-839B-25R-1, 33^41 cm) that has aZr/Hf ratio of 51 (cf. 38 from ICP-MS). These values are not significantlydifferent from the chondritic ratio of 36 (Jochum et al., 1986), although theICP-MS results show less variation and are generally closer to the chondriticvalue, suggesting that the ICP-MS Hf data may be more accurate. The INAAresults at high concentrations also have near-chondritic Zr/Hf (41-44). How-ever, the low INAA Hf results produce very high Zr/Hf (83-116), suggestingthat these Hf values are too low.

In addition to the comparisons made above, Cs, Rb, Sr, Zr, Nb, Y, W, U,Th, and Pb have all been measured by SSMS and either ICP-MS (Cs, Nb, W,U, and Th), XRF (Rb, Sr, Zr, and Y) or AA (Pb). Rb, Zr, and Y compare wellbetween the different techniques, with overall variations generally less than10%; Sr is generally within 20%. Th and U vary by as much as 70%, withSSMS U generally lower than ICP-MS values. W varies by as much as a factorof 3, with SSMS W consistently higher than ICP-MS W. Cs varies by a factorof 3 between SSMS and ICP-MS. Because XRF and SSMS Rb concentrationscompare well, the Rb/Cs ratio may be used to evaluate the reliability of the Csresults. Arc rocks have Rb/Cs ratios between 10 and 40 (Hart and Reid, 1991;

408


McDonough et al., 1992). The arc-related samples measured here have Rb/Cs= 27-143 for ICP-MS Cs or Rb/Cs = 22^8 for SSMS Cs. The single samplethat gave comparable Cs from SSMS and ICP-MS (135-839B-23R-1,0-6 cm,with Cs = 0.11 [ICM-PS] or 0.105 [SSMS]) has Rb/Cs = 37 (SSMS) or 45(ICM-PS). Based on these observations, the SSMS Cs would appear to be moreaccurate. Nb varies by as much as a factor of 1.8, with the SSMS data generally

higher than the ICP-MS data. Despite this variability, all six samples show highLa/Nb ratios, similar to arc basalts. Finally, Pb data vary by as much as a factorof 2.4. In general, the SSMS data are lower than the AA data, with results fromonly one sample falling within 10% of each other. The reason for this variabilityis not known, but if it is a result of contamination, then the lower values (i.e.,the SSMS data) would be the more accurate.

600

500

400

Ba

300

200

100

0200

Ba100

0100

Ba

Metis glass

Modern Tonganaxial volcanoes(excluding Ata)

O AtaSite 841dykes/sills

f\MetisW (whole rock)

200

Ba

100

King's TripleJunction

Kermadecs

200

Ba

100

0300

200h

o

D

o+×D

835836837838

Sites 835 to 838

-o-

• Site 834100

Ba. Modern

100 Tonganaxial '

volcanoes

Tonganboninitic suite

t . t

Site 834

5 10

% MgO15 0 10

% MgO15 20

Figure 12. Compilation of Ba-MgO data for the modem Lau Basin spreading centers, Sites 834-839, the Tonga-Kermadec modem axial volcanoes,and the northern Tonga boninites. The tie-line joins a coexisting whole rock-matrix pair.

Table 7. Selected element ratios within the various Leg 135 sites, compared with other volcanic provinces within the Lau-Tonga-Kermadec region.

Leg 135:Site 834 (>5% MgO)Site 835Site 836 (>5% MgO)Site 837Site 839

Lau Basin spreading centers:KTJ (>7% MgO)CLSC (>7% MgO)ELSC and ILSC (>7% MgO)Valu Fa (>7% MgO)

Niua fo'ou:(Oceanic Island Volcano)

Lau Ridge:LVG (>4% MgO)KVG (>4% MgO)

Tonga Ridge:North Tonga-Station 21Tafahi IslandFonualei IslandLate IslandMetis ShoalKao IslandTofua IslandHunga Ha'apai'Ata

Kermadec Ridge:Raoul GroupMacauley IslandL'Esperance

Rb/Cs

5.29^5.023.6-26.7

92-180(31.3)88.8-12625.6-74.0

—39.7-77.0

—37.1-48.6

67.5-85.0

——

20.941.5-61.110.0-53.8

17.742.544.730.8

28.5-95.9

20.9-40.933.1

24.8-39.4

K/Nb

603-22143219-36721012-25842151-29785747-13559

229-1845208-2352218-1449

6641-19647

381^42

19925811

1785-53132649

4300-8528762115897

2283-5798141623832—

2398-58115283-54443605-6074

Ba/La

2.06-10.111.1-13.87.91-23.816.7-16.819.7-38.0

3.57-11.83.98-13.94.43-13.532.5-48.8

8.0-10.5

19.8-26.232.9

28 6-59 945.6-16035.5-90.035.6-89.3116-124

44.374.9-76.478.3-98.437.5-47.7

35.6-68.341.3-72.234.8-70.5

La/Nb

1.23-3.133.85^.821.56-2.462.54-2.604.20-8.13

1.25-1.980.65-6.780.98-2.843.35-14.7

1.14-1.27

24

1.56-3.942.23

2.37-3.273.0-3.87

6.667.756.683.46—

1.62-5.895.59-6.394.21-7.61

Zr/Ba

1.59-15.00.77-1.670.88-5.370.78-0.930.38-0.80

1.37-3.171.45-7.511.49-7.850.26-1.19

2.45-3.23

0.46-0.910.14-0.15

0.087-0.180.12-0.290.15-0.210.19-0.28

0.130.38-0.420.19-0.250.17-0.240.23-0.32

0.26-0.830.23-0.440.23-0.32

Zr/Sm

26.4-35.517.4-22.529.8-47.317.7-17.813.2-21.6

19.1-31.315.7-27.018.3-25.918.1-22.7

26.0-34.5

15.0-18.78.84-8.97

9.86-17.015.7-18.417.7-21.315.8-21.5

24.518.9

17.6-20.415.4-27.518.7-24.5

14.8-23.113.0-16.915.3-19.5

Sr/Nd

12.4-20.118.9-25.028.8-50.712.5-12.825.7-38.1

14.8-15.510.3-36.710.5-37.432.4-37.8

12.5-18.3

22.9-33.142.8-51.8

35.2-78 873.2-11629.5^18.543.0-95.824.8-26.1

27.429.5-36.942.6-57.033.5-45.7

21.7-58.131.5-93.828.1-39.8

Sm/Hf

1.01-1.541.76-2.020.76-1.241.60-1.611.73-2.15

1.35-1.991.40-2.051.26-1.781.94-2.00

1.07-1.65

2.05-2.272.44-3.55

1.04-2.562.21

1.77-1.921.65-1.72

1.361.651.671.71

1.65-2.39

1.53-2.251.97-2.021.80-1.94

Ce/Sm

2.76-3.622.43-2.612.42-3.083.11-3.122.29-3.65

3.29-3.731.52-3.572.14-3.282.21-4.02

3.36-4.48

4.1-4.462.51-3.59

4.42-9.103.0-3.72

3.66-4.113.23-3.623.85-4.69

3.112.84

2.94-3.293.63-4.92

2.63-3.612.85^1.953.41_4.48

Ce/Pb

6.90-14.17.15-7.882.33-3.298.84-9.172.77-9.47

—4.12-36.63.46-11.5

—

9.46-12.2

——

1.0-1.92.30-2.901.77-2.69

1.635.3

2.252.27^.210.74-1.29

3.3-4.081.82-5.723.11-5.53

Nb/Th

3.25-22.92.72^4.73

82-1064.49-5.361.44-3.30

9.66.77-32.9

3.0-16.20.58-2.38

12.2-17.6

4.294.54

1.22-3.233.13

1.43^.342.651.571.741.703.71—

1.29-2.141.30-1.690.80-1.73

Nb/Nb(MORB)

0.39-1.060.21-0.220.18-0.450.79-0.83

0.077-0.33

0.38-1.240.26-3.000.26-0.920.13-O.17

1.59-1.94

1.290.86

(0.43-0.86)*0.20

0.47-0.600.260.21

0.17-0.520.150.22—

0.17-0.490.094-0.26

0.18-0.33

Zr/Zr(MORB)

1.05-2.010.59-0.660.49-0.81

1.0-1.030.35-0.73

0.72-1.070.47-1.010.46-0.850.32-0.50

1.26-1.62

1.16-0.890.35-0.51

0.12-0.310.19-0.200.50-0.690.31-0.39

0.620.63-0.690.58-0.720.32-0.410.54-0.76

0.22-0.930.16-0.540.50-0.61

EWA

RT, W

.B. B

RY

AN

, B.W

. C

X>

PI

TOrTor—<

ö

n

Notes: Data based on Tables 2-5 and published data (see text). Nb/Nb(MORB) and Zr/Zr(MORB) are N-MORB normalized, based on values from Sun and McDonough (1989). Value marked by an asterisk (*) was determined by X-rayfluorescence (XRF). KTJ = King's Triple Junction, CLSC = Central Lau Spreading Center, ELSC = Eastern Lau Spreading Center, ILSC = Intermediate Lau Spreading Center, LVG = Lau Volcanic Group, and KVG = KorobasagaVolcanic Group. Tafahi, Late, Kao, Tofua, Hunga Ha'apai, and L'Esperance islands composed of basaltic andesites; North Tonga-Station 21 composed of boninitic lavas; Fonualei Island composed of andesites and dacites; Metis Shoalcomposed of dacite/rhyolite; 'Ata and Raoul Group composed of basalts and basaltic andesites; and Macauley Island composed of basalts.


200

Zr

100

100

Zr

o200

Zr

100

0

200

Zr

100

Valu Fa

Site 839

Zr

500

400

Valu Fa

300

ELSCandlLSC

Sites 8 3 5 ^ "to 838

KermadecsModern Tongan axialvolcanoes (excluding

Kao and Ata)O Ata

100

King's Triple Junction

oSite 834Sites 835

to 838

• Tongan boninitic suite

100r

Site 839

Tonga Main Trend1 1 0

0 5 10

% MgO

Figure 13. Compilation of Zr-MgO data, as in Figure 12.

15 0

-

^ ( U n i t

J _

1

fr —

lUnits 3

Site

& 6

839

10

% MgO15 20

411


3 0 0

Ni

200

150

100

50

150

100

Ni50

ELSC andILSC

Kermadecs

150

100

Ni50

• Site 834

600

Ni

500

Modern Tonganaxial volcanoes

400

300

200

100

•' Valu Fa

Kermadecs

Tonganboninite suite

Site 839

0 5 10 15 0% MgO

Figure 14. Compilation of Ni-MgO data, as in Figure 12.

5 10% MgO

15 20

412


Site 839 - Units 5,7,and 9

MgO 4.1-5.8

Site 836 - Units 2(part) and 4MgO >7%

CsRbBaThUNbTa KLaCePbSrNdZrHfSmEuTi YYbLu

Figure 15. Element abundances of lavas from Sites 834-839 normalized toN-MORB values of Sun and McDonough (1989). Data from shipboard analy-ses (Parson, Hawkins, Allan, et al., 1992) and Tables 2-5. Only the moremagnesian lavas are included. The open circles represent what are inferred tobe outlying values. The heavy continuous line represent the N-MORB valuesof Hofmann (1988), whereas the heavy dashed lines are the minimum Nl-MORB values of Viereck et al. (1989).

413


Niua Fo'ou

MgO>6%

0 Central Lau Spreading Center

MgO>7%

East and Intermediate Lau Spreading Centers

MgO >7%

CsRbBaTh U NbTa K LaCe PbSrNdZrHfSmEuTi Y YbLuFigure 16. Element abundances of lavas from the Lau Basin spreading centers, and theisland of Niuafo'ou, normalized to N-MORB values. Only the most magnesian lavas areplotted. Data sources listed in text. N-MORB values and curves as in Figure 15.

414


N.Tonga boninitβs (station 21)

MgO>6.0%

Tafahi Basaltic Andesites

CsRbBaTh U NbTa K LaCePbSrNd Zr HfSmEuTi YYbLu

Figure 17. Element abundances of the modern Tofua Arc volcanoes (basaltic andesitesonly), and the northern Tonga boninites, from Station 21, normalized to N-MORBvalues. Normalizing factors and curves as in Figure 15.

415


40

L'Esperaπce

MgO 4.47 % - 4.97%

Macauley Is* MgO >8.0 %• MgO 4.7 - 7.25 %

0.1

Cs Rb Ba Th U Nb Ta K La Ce Pb Sr Nd Zr Hf Sm Eu Ti Y Yb Lu

Figure 18. Element abundances of the modern Kermadec volcanoes (basalts and basaltic andesites only),

normalized to N-MORB. Normalizing factors as in Figure 15.

416


Korobasaga Volcanic Group, Lau Is;

4.4 - 2.4 Ma.MgO 4.3 - 5.1 %

Lau Volcanic Group, Lau Is;

14.0 - 5.4 Ma.MgO 4.17 - 4.36

Rb Ba Th Nb Ta K La Ce Sr Nd Zr Hf Sm Eu Ti Y Yb Lu

Figure 19. Element abundances of the Lau and Korobasaga Volcanic Groups, Lau Islands (after Cole et al., 1990),normalized to N-MORB.

417


Site 837 - Andesites

10

L'EsperanceBasaltic Andesites

20 r Macauley basalts

Raoul Basalts andBasaltic Andesites

Basaltic Andesites

Unit 9Site 839

Unit 1

Unit 3

Site 834 - Units 2 to 13

I I I I I I I L i I I i I

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb LuFigure 20. Chondrite normalized REE (Sun and McDonough, 1989) patterns for Sites834-839 lavas (Tables 2-5), the modern Tofua Arc volcanoes, Niuafo'ou, and themodern Kermadec Arc volcanoes.

418


Fractional Melting ModelSpinel - pyroxene peridotite

Model 1

Initial w t fractions

Wt fraction entering <quid

Rb Ba Sr Th Nb La Ce Nd Zr Hf Sm Eu Ti Yb LuK

Figure 21. Results of four sets of calculations showing the progressive change of selected trace andminor element compositions in melts from two model peridotite sources undergoing fractionalmelting. Melt compositions normalized to starting source composition, and based on increments ofmelt fractions (= F) as shown. Models 1 and 3 based on spinel-pyroxene peridotite. Models 2 and 4based on amphibole peridotite. Models 1 and 3 and Models 2 and 4 differ with respect to the partitioncoefficients used (listed in Table 6). Initial melt, melt increments, and melt remaining in residue areall taken as 0.01 (= melt fractions).

419


0.1

Fractional Melting ModelAmphibole Peridotite

Model 2

Initial Melt 0.01Melt Increment 0.01Melt remainingin residue 0.01 Initial Wt fractions

Wt fractions entering liquid (1)(2)

Ollv0.5090.150.2

Opx

0.2470.150.2

Cpx

0.0380.35

Amph0.1160.350.6


100r

0.01 -

0.05

Fractional Melting ModelSpinel - pyroxene Peridotite

Model 3


Figure 21 (continued).

420

0.01 -

0.004


Fractional Melting ModelAmphibole Peridotite

Model 4


Figure 21 (continued).


gI - 0.15

= T 0.05

XLSC/Limit

Sites 835 and836 Limit

Northern TongaStation 21 boninites

j I i j i

0 0.2 0.4 O.β 0.8 1.0 I 2 4 β 8 10 12 14

Zr / Ba

Figure 22. Comparison of the compositional fields, in terms of TiO2/CaO vs. Zr/Ba ratios, of Sites 834-836 and 839 lavas with the Lau Basin spreading

centers, the modern Tonga-Kermadec volcanoes, and the northern Tonga boninites.

O 0 20to

OD

+

o

•

SITE

SITE

SITE

SITE

834

835

836

839

KTJ

Northern Tonga

Station 21 boninites,northern Tonga

50 60

Sr / Nd

Figure 23. Compositional fields, in terms of TiO2/CaO and Sr/Nd ratios, of Sites 834-836 and 839 lavas with those of the Lau Basin spreading centers,

the modern Tonga-Kermadec arc volcanoes, and the northern Tonga boninites.


D

+

O

•

SITE

SITE

SITE

SITE

834

835

836

839

008 0 1 I 0

Zr / BaI0 20

Figure 24. Zr/Sm vs. Zr/Ba ratios for the Lau-Tonga-Kermadec lavas. The effects of fractionation of clinopyroxene, olivine, and Plagioclase (to F = 0.5) are shownby the vectors.

423


Zr / Ba

Figure 25. Simplified map of the Lau-Tonga region showing the regionalvariations and ranges of Zr/Ba ratios within the lavas, including those recov-ered during Leg 135. Based on data presented in Table 7.

1

Former Lau-Tonga single ridge

~ 14 - 10MaLau Volcanic Group


Initial opening-6 - 10 Ma o' L a u B a s i n

Lau Ridge Tonga RidgeSite 834

/ Suggested subduction

jump at ~β - 9 Ma

Influx of new meltand mantle (progressively from north ?)

? Proto Tofua arc (Site 8 3 9 )

Lau^idge '***%«$- I Tonga Ridgestyle rifting

Figure 26. A sequence of four highly speculative east-west sections, in the region of about latitudes 18c-20°S, suggesting mechanisms by which the Lau-Tonga

systems may have evolved. The stippling shows the inferred extent of the subduction-derived component into the mantle wedge.

425

Documents

24. REGIONAL GEOCHEMISTRY OF THE LAU …...more typical seafloor spreading fabric. The latter region is interpreted as having formed from the still active propagating spreading ridges